On Tue, Apr 20, 2021 at 7:20 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 4/15/21 1:40 AM, Muchun Song wrote:
> > When we free a HugeTLB page to the buddy allocator, we need to allocate
> > the vmemmap pages associated with it. However, we may not be able to
> > allocate the vmemmap pages when the system is under memory pressure. In
> > this case, we just refuse to free the HugeTLB page. This changes behavior
> > in some corner cases as listed below:
> >
> > 1) Failing to free a huge page triggered by the user (decrease nr_pages).
> >
> > User needs to try again later.
> >
> > 2) Failing to free a surplus huge page when freed by the application.
> >
> > Try again later when freeing a huge page next time.
> >
> > 3) Failing to dissolve a free huge page on ZONE_MOVABLE via
> > offline_pages().
> >
> > This can happen when we have plenty of ZONE_MOVABLE memory, but
> > not enough kernel memory to allocate vmemmmap pages. We may even
> > be able to migrate huge page contents, but will not be able to
> > dissolve the source huge page. This will prevent an offline
> > operation and is unfortunate as memory offlining is expected to
> > succeed on movable zones. Users that depend on memory hotplug
> > to succeed for movable zones should carefully consider whether the
> > memory savings gained from this feature are worth the risk of
> > possibly not being able to offline memory in certain situations.
> >
> > 4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
> > alloc_contig_range() - once we have that handling in place. Mainly
> > affects CMA and virtio-mem.
> >
> > Similar to 3). virito-mem will handle migration errors gracefully.
> > CMA might be able to fallback on other free areas within the CMA
> > region.
> >
> > Vmemmap pages are allocated from the page freeing context. In order for
> > those allocations to be not disruptive (e.g. trigger oom killer)
> > __GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
> > because a non sleeping allocation would be too fragile and it could fail
> > too easily under memory pressure. GFP_ATOMIC or other modes to access
> > memory reserves is not used because we want to prevent consuming
> > reserves under heavy hugetlb freeing.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> > Documentation/admin-guide/mm/hugetlbpage.rst | 8 +++
> > Documentation/admin-guide/mm/memory-hotplug.rst | 13 ++++
> > include/linux/hugetlb.h | 3 +
> > include/linux/mm.h | 2 +
> > mm/hugetlb.c | 85 ++++++++++++++++++++-----
> > mm/hugetlb_vmemmap.c | 34 ++++++++++
> > mm/hugetlb_vmemmap.h | 6 ++
> > mm/sparse-vmemmap.c | 75 +++++++++++++++++++++-
> > 8 files changed, 210 insertions(+), 16 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> > index f7b1c7462991..6988895d09a8 100644
> > --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> > +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> > @@ -60,6 +60,10 @@ HugePages_Surp
> > the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
> > maximum number of surplus huge pages is controlled by
> > ``/proc/sys/vm/nr_overcommit_hugepages``.
> > + Note: When the feature of freeing unused vmemmap pages associated
> > + with each hugetlb page is enabled, the number of surplus huge pages
> > + may be temporarily larger than the maximum number of surplus huge
> > + pages when the system is under memory pressure.
> > Hugepagesize
> > is the default hugepage size (in Kb).
> > Hugetlb
> > @@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task. A user with root
> > privileges can dynamically allocate more or free some persistent huge pages
> > by increasing or decreasing the value of ``nr_hugepages``.
> >
> > +Note: When the feature of freeing unused vmemmap pages associated with each
> > +hugetlb page is enabled, we can fail to free the huge pages triggered by
> > +the user when ths system is under memory pressure. Please try again later.
> > +
> > Pages that are used as huge pages are reserved inside the kernel and cannot
> > be used for other purposes. Huge pages cannot be swapped out under
> > memory pressure.
> > diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
> > index 05d51d2d8beb..c6bae2d77160 100644
> > --- a/Documentation/admin-guide/mm/memory-hotplug.rst
> > +++ b/Documentation/admin-guide/mm/memory-hotplug.rst
> > @@ -357,6 +357,19 @@ creates ZONE_MOVABLE as following.
> > Unfortunately, there is no information to show which memory block belongs
> > to ZONE_MOVABLE. This is TBD.
> >
> > + Memory offlining can fail when dissolving a free huge page on ZONE_MOVABLE
> > + and the feature of freeing unused vmemmap pages associated with each hugetlb
> > + page is enabled.
> > +
> > + This can happen when we have plenty of ZONE_MOVABLE memory, but not enough
> > + kernel memory to allocate vmemmmap pages. We may even be able to migrate
> > + huge page contents, but will not be able to dissolve the source huge page.
> > + This will prevent an offline operation and is unfortunate as memory offlining
> > + is expected to succeed on movable zones. Users that depend on memory hotplug
> > + to succeed for movable zones should carefully consider whether the memory
> > + savings gained from this feature are worth the risk of possibly not being
> > + able to offline memory in certain situations.
> > +
> > .. note::
> > Techniques that rely on long-term pinnings of memory (especially, RDMA and
> > vfio) are fundamentally problematic with ZONE_MOVABLE and, therefore, memory
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 0abed7e766b8..6e970a7d3480 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -525,6 +525,7 @@ unsigned long hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
> > * code knows it has only reference. All other examinations and
> > * modifications require hugetlb_lock.
> > * HPG_freed - Set when page is on the free lists.
> > + * HPG_vmemmap_optimized - Set when the vmemmap pages of the page are freed.
> > * Synchronization: hugetlb_lock held for examination and modification.
>
> I like the per-page flag. In previous versions of the series, you just
> checked the free_vmemmap_pages_per_hpage() to determine if vmemmmap
> should be allocated. Is there any change in functionality that makes is
> necessary to set the flag in each page, or is it mostly for flexibility
> going forward?
Actually, only the routine of dissolving the page cares whether
the page is on the buddy free list when update_and_free_page
returns. But we cannot change the return type of the
update_and_free_page (e.g. change return type from 'void' to 'int').
Why? If the hugepage is freed through a kworker, we cannot
know the return value when update_and_free_page returns.
So adding a return value seems odd.
In the dissolving routine, We can allocate vmemmap pages first,
if it is successful, then we can make sure that
update_and_free_page can successfully free page. So I need
some stuff to mark the page which does not need to allocate
vmemmap pages.
On the surface, we seem to have a straightforward method
to do this.
Add a new parameter 'alloc_vmemmap' to update_and_free_page() to
indicate that the caller is already allocated the vmemmap pages.
update_and_free_page() do not need to allocate. Just like below.
void update_and_free_page(struct hstate *h, struct page *page, bool atomic,
bool alloc_vmemmap)
{
if (alloc_vmemmap)
// allocate vmemmap pages
}
But if the page is freed through a kworker. How to pass
'alloc_vmemmap' to the kworker? We can embed this
information into the per-page flag. So if we introduce
HPG_vmemmap_optimized, the parameter of
alloc_vmemmap is also necessary.
So it seems that introducing HPG_vmemmap_optimized is
a good choice.
>
> > */
> > enum hugetlb_page_flags {
> > @@ -532,6 +533,7 @@ enum hugetlb_page_flags {
> > HPG_migratable,
> > HPG_temporary,
> > HPG_freed,
> > + HPG_vmemmap_optimized,
> > __NR_HPAGEFLAGS,
> > };
> >
> > @@ -577,6 +579,7 @@ HPAGEFLAG(RestoreReserve, restore_reserve)
> > HPAGEFLAG(Migratable, migratable)
> > HPAGEFLAG(Temporary, temporary)
> > HPAGEFLAG(Freed, freed)
> > +HPAGEFLAG(VmemmapOptimized, vmemmap_optimized)
> >
> > #ifdef CONFIG_HUGETLB_PAGE
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index a4d160ddb749..d0854828bb9c 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3048,6 +3048,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
> >
> > void vmemmap_remap_free(unsigned long start, unsigned long end,
> > unsigned long reuse);
> > +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > + unsigned long reuse, gfp_t gfp_mask);
> >
> > void *sparse_buffer_alloc(unsigned long size);
> > struct page * __populate_section_memmap(unsigned long pfn,
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index eeb8f5480170..1c37f0098e00 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1376,6 +1376,34 @@ static void remove_hugetlb_page(struct hstate *h, struct page *page,
> > h->nr_huge_pages_node[nid]--;
> > }
> >
> > +static void add_hugetlb_page(struct hstate *h, struct page *page,
> > + bool adjust_surplus)
> > +{
>
> We need to be a bit careful with hugepage specific flags that may be
> set. The routine remove_hugetlb_page which is called for 'page' before
> this routine will not clear any of the hugepage specific flags. If the
> calling path goes through free_huge_page, most but not all flags are
> cleared.
>
> We had a discussion about clearing the page->private field in Oscar's
> series. In the case of 'new' pages we can assume page->private is
> cleared, but perhaps we should not make that assumption here. Since we
> hope to rarely call this routine, it might be safer to do something
> like:
>
> set_page_private(page, 0);
> SetHPageVmemmapOptimized(page);
Agree. Thanks for your reminder. I will fix this.
>
> > + int nid = page_to_nid(page);
> > +
> > + lockdep_assert_held(&hugetlb_lock);
> > +
> > + INIT_LIST_HEAD(&page->lru);
> > + h->nr_huge_pages++;
> > + h->nr_huge_pages_node[nid]++;
> > +
> > + if (adjust_surplus) {
> > + h->surplus_huge_pages++;
> > + h->surplus_huge_pages_node[nid]++;
> > + }
> > +
> > + set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> > +
> > + /*
> > + * The refcount can possibly be increased by memory-failure or
> > + * soft_offline handlers.
> > + */
> > + if (likely(put_page_testzero(page))) {
>
> In the existing code there is no such test. Is the need for the test
> because of something introduced in the new code?
No.
> Or, should this test be in the existing code?
Yes. gather_surplus_pages should be fixed. I can fix it
in a separate patch.
The possible bad scenario:
CPU0: CPU1:
set_compound_page_dtor(HUGETLB_PAGE_DTOR);
memory_failure_hugetlb
get_hwpoison_page
__get_hwpoison_page
get_page_unless_zero
put_page_testzero()
put_page(page)
More details and discussion can refer to:
https://lore.kernel.org/linux-doc/CAMZfGtVRSBkKe=tKAKLY8dp_hywotq3xL+EJZNjXuSKt3HK3bQ@xxxxxxxxxxxxxx/
>
> Sorry, I am not seeing why this is needed.
>
> > + arch_clear_hugepage_flags(page);
> > + enqueue_huge_page(h, page);
> > + }
> > +}
> > +
> > static void __update_and_free_page(struct hstate *h, struct page *page)
> > {
> > int i;
> > @@ -1384,6 +1412,18 @@ static void __update_and_free_page(struct hstate *h, struct page *page)
> > if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> > return;
> >
> > + if (alloc_huge_page_vmemmap(h, page)) {
> > + spin_lock_irq(&hugetlb_lock);
> > + /*
> > + * If we cannot allocate vmemmap pages, just refuse to free the
> > + * page and put the page back on the hugetlb free list and treat
> > + * as a surplus page.
> > + */
> > + add_hugetlb_page(h, page, true);
> > + spin_unlock_irq(&hugetlb_lock);
> > + return;
> > + }
> > +
> > for (i = 0; i < pages_per_huge_page(h);
> > i++, subpage = mem_map_next(subpage, page, i)) {
> > subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
> > @@ -1444,7 +1484,7 @@ static inline void flush_free_hpage_work(struct hstate *h)
> > static void update_and_free_page(struct hstate *h, struct page *page,
> > bool atomic)
> > {
> > - if (!free_vmemmap_pages_per_hpage(h) || !atomic) {
> > + if (!HPageVmemmapOptimized(page) || !atomic) {
> > __update_and_free_page(h, page);
> > return;
> > }
>
> When update_and_free_pages_bulk was added it was done to avoid
> lock/unlock cycles with each page. At the time, I thought about the
> addition of code to allocate vmmemmap, and the possibility that those
> allocations could fail. I thought it might make sense to perhaps
> process the pages one at a time so that we could quit at the first
> allocation failure. After more thought, I think it is best to leave the
> code to do bulk operations as you have done above. Why?
> - Just because one allocation fails does not mean the next will fail.
> It is possible the allocations could be from different nodes/zones.
> - We will still need to put the requested number of pages into surplus
> state.
>
> I am not suggesting you change anything. Just wanted to share my
> thoughts in case someone thought otherwise.
>
> > @@ -1790,10 +1830,14 @@ static struct page *remove_pool_huge_page(struct hstate *h,
> > * nothing for in-use hugepages and non-hugepages.
> > * This function returns values like below:
> > *
> > - * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> > - * (allocated or reserved.)
> > - * 0: successfully dissolved free hugepages or the page is not a
> > - * hugepage (considered as already dissolved)
> > + * -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
> > + * when the system is under memory pressure and the feature of
> > + * freeing unused vmemmap pages associated with each hugetlb page
> > + * is enabled.
> > + * -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> > + * (allocated or reserved.)
> > + * 0: successfully dissolved free hugepages or the page is not a
> > + * hugepage (considered as already dissolved)
> > */
> > int dissolve_free_huge_page(struct page *page)
> > {
> > @@ -1835,19 +1879,30 @@ int dissolve_free_huge_page(struct page *page)
> > goto retry;
> > }
> >
> > - /*
> > - * Move PageHWPoison flag from head page to the raw error page,
> > - * which makes any subpages rather than the error page reusable.
> > - */
> > - if (PageHWPoison(head) && page != head) {
> > - SetPageHWPoison(page);
> > - ClearPageHWPoison(head);
> > - }
> > remove_hugetlb_page(h, page, false);
> > h->max_huge_pages--;
> > spin_unlock_irq(&hugetlb_lock);
> > - update_and_free_page(h, head, false);
> > - return 0;
> > +
> > + rc = alloc_huge_page_vmemmap(h, page);
> > + if (!rc) {
> > + /*
> > + * Move PageHWPoison flag from head page to the raw
> > + * error page, which makes any subpages rather than
> > + * the error page reusable.
> > + */
> > + if (PageHWPoison(head) && page != head) {
> > + SetPageHWPoison(page);
> > + ClearPageHWPoison(head);
> > + }
> > + update_and_free_page(h, head, false);
> > + } else {
> > + spin_lock_irq(&hugetlb_lock);
> > + add_hugetlb_page(h, page, false);
> > + h->max_huge_pages++;
> > + spin_unlock_irq(&hugetlb_lock);
> > + }
> > +
> > + return rc;
> > }
> > out:
> > spin_unlock_irq(&hugetlb_lock);
>
> Changes in the files below have not changed in any significant way
> since the previous version. The code looks good to me, but I would
> like to see if there are comments from others.
Thanks for your review. :-)
>
> Thanks,
> --
> Mike Kravetz
>
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index cb28c5b6c9ff..a897c7778246 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -185,6 +185,38 @@ static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> > return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> > }
> >
> > +/*
> > + * Previously discarded vmemmap pages will be allocated and remapping
> > + * after this function returns zero.
> > + */
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > + int ret;
> > + unsigned long vmemmap_addr = (unsigned long)head;
> > + unsigned long vmemmap_end, vmemmap_reuse;
> > +
> > + if (!HPageVmemmapOptimized(head))
> > + return 0;
> > +
> > + vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > + vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > + vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > + /*
> > + * The pages which the vmemmap virtual address range [@vmemmap_addr,
> > + * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> > + * the range is mapped to the page which @vmemmap_reuse is mapped to.
> > + * When a HugeTLB page is freed to the buddy allocator, previously
> > + * discarded vmemmap pages must be allocated and remapping.
> > + */
> > + ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> > + GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
> > +
> > + if (!ret)
> > + ClearHPageVmemmapOptimized(head);
> > +
> > + return ret;
> > +}
> > +
> > void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > {
> > unsigned long vmemmap_addr = (unsigned long)head;
> > @@ -203,4 +235,6 @@ void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > * which the range [@vmemmap_addr, @vmemmap_end] is mapped to.
> > */
> > vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
> > +
> > + SetHPageVmemmapOptimized(head);
> > }
> > diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> > index 01f8637adbe0..a37771b0b82a 100644
> > --- a/mm/hugetlb_vmemmap.h
> > +++ b/mm/hugetlb_vmemmap.h
> > @@ -11,6 +11,7 @@
> > #include <linux/hugetlb.h>
> >
> > #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
> > void free_huge_page_vmemmap(struct hstate *h, struct page *head);
> >
> > /*
> > @@ -25,6 +26,11 @@ static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> > return 0;
> > }
> > #else
> > +static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > + return 0;
> > +}
> > +
> > static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > {
> > }
> > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> > index 7d40b5bd7046..693de0aec7a8 100644
> > --- a/mm/sparse-vmemmap.c
> > +++ b/mm/sparse-vmemmap.c
> > @@ -40,7 +40,8 @@
> > * @remap_pte: called for each lowest-level entry (PTE).
> > * @reuse_page: the page which is reused for the tail vmemmap pages.
> > * @reuse_addr: the virtual address of the @reuse_page page.
> > - * @vmemmap_pages: the list head of the vmemmap pages that can be freed.
> > + * @vmemmap_pages: the list head of the vmemmap pages that can be freed
> > + * or is mapped from.
> > */
> > struct vmemmap_remap_walk {
> > void (*remap_pte)(pte_t *pte, unsigned long addr,
> > @@ -224,6 +225,78 @@ void vmemmap_remap_free(unsigned long start, unsigned long end,
> > free_vmemmap_page_list(&vmemmap_pages);
> > }
> >
> > +static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> > + struct vmemmap_remap_walk *walk)
> > +{
> > + pgprot_t pgprot = PAGE_KERNEL;
> > + struct page *page;
> > + void *to;
> > +
> > + BUG_ON(pte_page(*pte) != walk->reuse_page);
> > +
> > + page = list_first_entry(walk->vmemmap_pages, struct page, lru);
> > + list_del(&page->lru);
> > + to = page_to_virt(page);
> > + copy_page(to, (void *)walk->reuse_addr);
> > +
> > + set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> > +}
> > +
> > +static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
> > + gfp_t gfp_mask, struct list_head *list)
> > +{
> > + unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
> > + int nid = page_to_nid((struct page *)start);
> > + struct page *page, *next;
> > +
> > + while (nr_pages--) {
> > + page = alloc_pages_node(nid, gfp_mask, 0);
> > + if (!page)
> > + goto out;
> > + list_add_tail(&page->lru, list);
> > + }
> > +
> > + return 0;
> > +out:
> > + list_for_each_entry_safe(page, next, list, lru)
> > + __free_pages(page, 0);
> > + return -ENOMEM;
> > +}
> > +
> > +/**
> > + * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
> > + * to the page which is from the @vmemmap_pages
> > + * respectively.
> > + * @start: start address of the vmemmap virtual address range that we want
> > + * to remap.
> > + * @end: end address of the vmemmap virtual address range that we want to
> > + * remap.
> > + * @reuse: reuse address.
> > + * @gpf_mask: GFP flag for allocating vmemmap pages.
> > + */
> > +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> > + unsigned long reuse, gfp_t gfp_mask)
> > +{
> > + LIST_HEAD(vmemmap_pages);
> > + struct vmemmap_remap_walk walk = {
> > + .remap_pte = vmemmap_restore_pte,
> > + .reuse_addr = reuse,
> > + .vmemmap_pages = &vmemmap_pages,
> > + };
> > +
> > + /* See the comment in the vmemmap_remap_free(). */
> > + BUG_ON(start - reuse != PAGE_SIZE);
> > +
> > + might_sleep_if(gfpflags_allow_blocking(gfp_mask));
> > +
> > + if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages))
> > + return -ENOMEM;
> > +
> > + vmemmap_remap_range(reuse, end, &walk);
> > +
> > + return 0;
> > +}
> > +
> > /*
> > * Allocate a block of memory to be used to back the virtual memory map
> > * or to back the page tables that are used to create the mapping.
> >
