On Wed, Oct 26, 2022 at 04:54:01PM -0700, Mike Kravetz wrote:
> On 10/26/22 17:42, Peter Xu wrote:
> > Hi, Mike,
> >
> > On Sat, Oct 22, 2022 at 07:50:47PM -0700, Mike Kravetz wrote:
> >
> > [...]
> >
> > > -void __unmap_hugepage_range_final(struct mmu_gather *tlb,
> > > +static void __unmap_hugepage_range_locking(struct mmu_gather *tlb,
> > > struct vm_area_struct *vma, unsigned long start,
> > > unsigned long end, struct page *ref_page,
> > > - zap_flags_t zap_flags)
> > > + zap_flags_t zap_flags, bool final)
> > > {
> > > hugetlb_vma_lock_write(vma);
> > > i_mmap_lock_write(vma->vm_file->f_mapping);
> > >
> > > __unmap_hugepage_range(tlb, vma, start, end, ref_page, zap_flags);
> > >
> > > - /*
> > > - * Unlock and free the vma lock before releasing i_mmap_rwsem. When
> > > - * the vma_lock is freed, this makes the vma ineligible for pmd
> > > - * sharing. And, i_mmap_rwsem is required to set up pmd sharing.
> > > - * This is important as page tables for this unmapped range will
> > > - * be asynchrously deleted. If the page tables are shared, there
> > > - * will be issues when accessed by someone else.
> > > - */
> > > - __hugetlb_vma_unlock_write_free(vma);
> > > + if (final) {
> > > + /*
> > > + * Unlock and free the vma lock before releasing i_mmap_rwsem.
> > > + * When the vma_lock is freed, this makes the vma ineligible
> > > + * for pmd sharing. And, i_mmap_rwsem is required to set up
> > > + * pmd sharing. This is important as page tables for this
> > > + * unmapped range will be asynchrously deleted. If the page
> > > + * tables are shared, there will be issues when accessed by
> > > + * someone else.
> > > + */
> > > + __hugetlb_vma_unlock_write_free(vma);
> > > + i_mmap_unlock_write(vma->vm_file->f_mapping);
> >
> > Pure question: can we rely on hugetlb_vm_op_close() to destroy the hugetlb
> > vma lock?
> >
> > I read the comment above, it seems we are trying to avoid racing with pmd
> > sharing, but I don't see how that could ever happen, since iiuc there
> > should only be two places that unmaps the vma (final==true):
> >
> > (1) munmap: we're holding write lock, so no page fault possible
> > (2) exit_mmap: we've already reset current->mm so no page fault possible
> >
>
> Thanks for taking a look Peter!
>
> The possible sharing we are trying to stop would be initiated by a fault
> in a different process on the same underlying mapping object (inode). The
> specific vma in exit processing is still linked into the mapping interval
> tree. So, even though we call huge_pmd_unshare in the unmap processing (in
> __unmap_hugepage_range) the sharing could later be initiated by another
> process.
>
> Hope that makes sense. That is also the reason the routine
> page_table_shareable contains this check:
>
> /*
> * match the virtual addresses, permission and the alignment of the
> * page table page.
> *
> * Also, vma_lock (vm_private_data) is required for sharing.
> */
> if (pmd_index(addr) != pmd_index(saddr) ||
> vm_flags != svm_flags ||
> !range_in_vma(svma, sbase, s_end) ||
> !svma->vm_private_data)
> return 0;
Ah, makes sense. Hmm, then I'm wondering whether hugetlb_vma_lock_free()
would ever be useful at all? Because remove_vma() (or say, the close()
hook) seems to always be called after an precedent unmap_vmas().
>
> FYI - The 'flags' check also prevents a non-uffd mapping from initiating
> sharing with a uffd mapping.
>
> > > + } else {
> > > + i_mmap_unlock_write(vma->vm_file->f_mapping);
> > > + hugetlb_vma_unlock_write(vma);
> > > + }
> > > +}
> > >
> > > - i_mmap_unlock_write(vma->vm_file->f_mapping);
> > > +void __unmap_hugepage_range_final(struct mmu_gather *tlb,
> > > + struct vm_area_struct *vma, unsigned long start,
> > > + unsigned long end, struct page *ref_page,
> > > + zap_flags_t zap_flags)
> > > +{
> > > + __unmap_hugepage_range_locking(tlb, vma, start, end, ref_page,
> > > + zap_flags, true);
> > > }
> > >
> > > +#ifdef CONFIG_ADVISE_SYSCALLS
> > > +/*
> > > + * Similar setup as in zap_page_range(). madvise(MADV_DONTNEED) can not call
> > > + * zap_page_range for hugetlb vmas as __unmap_hugepage_range_final will delete
> > > + * the associated vma_lock.
> > > + */
> > > +void clear_hugetlb_page_range(struct vm_area_struct *vma, unsigned long start,
> > > + unsigned long end)
> > > +{
> > > + struct mmu_notifier_range range;
> > > + struct mmu_gather tlb;
> > > +
> > > + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
> > > + start, end);
> >
> > Is mmu_notifier_invalidate_range_start() missing here?
> >
>
> It certainly does look like it. When I created this routine, I was trying to
> mimic what was done in the current calling path zap_page_range to
> __unmap_hugepage_range_final. Now when I look at that, I am not seeing
> a mmu_notifier_invalidate_range_start/end. Am I missing something, or
> are these missing today?
I'm not sure whether we're looking at the same code base; here it's in
zap_page_range() itself.
mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
start, start + size);
tlb_gather_mmu(&tlb, vma->vm_mm);
update_hiwater_rss(vma->vm_mm);
mmu_notifier_invalidate_range_start(&range);
do {
unmap_single_vma(&tlb, vma, start, range.end, NULL);
} while ((vma = mas_find(&mas, end - 1)) != NULL);
mmu_notifier_invalidate_range_end(&range);
> Do note that we do MMU_NOTIFY_UNMAP in __unmap_hugepage_range.
Hmm, I think we may want CLEAR for zap-only and UNMAP only for unmap.
* @MMU_NOTIFY_UNMAP: either munmap() that unmap the range or a mremap() that
* move the range
* @MMU_NOTIFY_CLEAR: clear page table entry (many reasons for this like
* madvise() or replacing a page by another one, ...).
The other thing is that unmap_vmas() also notifies (same to
zap_page_range), it looks a duplicated notification if any of them calls
__unmap_hugepage_range() at last.
>
> > > + tlb_gather_mmu(&tlb, vma->vm_mm);
> > > + update_hiwater_rss(vma->vm_mm);
> > > +
> > > + __unmap_hugepage_range_locking(&tlb, vma, start, end, NULL, 0, false);
> > > +
> > > + mmu_notifier_invalidate_range_end(&range);
> > > + tlb_finish_mmu(&tlb);
> > > +}
> > > +#endif
> > > +
> > > void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
> > > unsigned long end, struct page *ref_page,
> > > zap_flags_t zap_flags)
> > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > index 2baa93ca2310..90577a669635 100644
> > > --- a/mm/madvise.c
> > > +++ b/mm/madvise.c
> > > @@ -790,7 +790,10 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
> > > static long madvise_dontneed_single_vma(struct vm_area_struct *vma,
> > > unsigned long start, unsigned long end)
> > > {
> > > - zap_page_range(vma, start, end - start);
> > > + if (!is_vm_hugetlb_page(vma))
> > > + zap_page_range(vma, start, end - start);
> > > + else
> > > + clear_hugetlb_page_range(vma, start, end);
> > > return 0;
> > > }
> >
> > This does look a bit unfortunate - zap_page_range() contains yet another
> > is_vm_hugetlb_page() check (further down in unmap_single_vma), it can be
> > very confusing on which code path is really handling hugetlb.
> >
> > The other mm_users check in v3 doesn't need this change, but was a bit
> > hackish to me, because IIUC we're clear on the call paths to trigger this
> > (unmap_vmas), so it seems clean to me to pass that info from the upper
> > stack.
> >
> > Maybe we can have a new zap_flags passed into unmap_single_vma() showing
> > that it's destroying the vma?
>
> I thought about that. However, we would need to start passing the flag
> here into zap_page_range as this is the beginning of that call down into
> the hugetlb code where we do not want to remove zap_page_rangethe
> vma_lock.
Right. I was thinking just attach the new flag in unmap_vmas(). A pesudo
(not compiled) code attached.
Thanks,
--
Peter Xu
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 978c17df053e..37091f8a6a12 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3463,5 +3463,6 @@ madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
* default, the flag is not set.
*/
#define ZAP_FLAG_DROP_MARKER ((__force zap_flags_t) BIT(0))
+#define ZAP_FLAG_UNMAP ((__force zap_flags_t) BIT(1))
#endif /* _LINUX_MM_H */
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 4a8c8456555e..245954d85553 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5201,8 +5201,10 @@ static void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct
static void __unmap_hugepage_range_locking(struct mmu_gather *tlb,
struct vm_area_struct *vma, unsigned long start,
unsigned long end, struct page *ref_page,
- zap_flags_t zap_flags, bool final)
+ zap_flags_t zap_flags)
{
+ bool final = zap_flags & ZAP_FLAG_UNMAP;
+
hugetlb_vma_lock_write(vma);
i_mmap_lock_write(vma->vm_file->f_mapping);
@@ -5232,7 +5234,7 @@ void __unmap_hugepage_range_final(struct mmu_gather *tlb,
zap_flags_t zap_flags)
{
__unmap_hugepage_range_locking(tlb, vma, start, end, ref_page,
- zap_flags, true);
+ zap_flags);
}
#ifdef CONFIG_ADVISE_SYSCALLS
@@ -5252,7 +5254,7 @@ void clear_hugetlb_page_range(struct vm_area_struct *vma, unsigned long start,
tlb_gather_mmu(&tlb, vma->vm_mm);
update_hiwater_rss(vma->vm_mm);
- __unmap_hugepage_range_locking(&tlb, vma, start, end, NULL, 0, false);
+ __unmap_hugepage_range_locking(&tlb, vma, start, end, NULL, 0);
mmu_notifier_invalidate_range_end(&range);
tlb_finish_mmu(&tlb);
diff --git a/mm/memory.c b/mm/memory.c
index c5599a9279b1..679b702af4ce 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1671,7 +1671,7 @@ void unmap_vmas(struct mmu_gather *tlb, struct maple_tree *mt,
{
struct mmu_notifier_range range;
struct zap_details details = {
- .zap_flags = ZAP_FLAG_DROP_MARKER,
+ .zap_flags = ZAP_FLAG_DROP_MARKER | ZAP_FLAG_UNMAP,
/* Careful - we need to zap private pages too! */
.even_cows = true,
};
