On 2024/5/8 16:53, Ryan Roberts wrote:
On 08/05/2024 04:44, Baolin Wang wrote:
On 2024/5/7 18:37, Ryan Roberts wrote:
On 06/05/2024 09:46, Baolin Wang wrote:
Add large folio mapping establishment support for finish_fault() as a
preparation,
to support multi-size THP allocation of anonymous shmem pages in the following
patches.
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
mm/memory.c | 43 +++++++++++++++++++++++++++++++++----------
1 file changed, 33 insertions(+), 10 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index eea6e4984eae..936377220b77 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4747,9 +4747,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
{
struct vm_area_struct *vma = vmf->vma;
struct page *page;
+ struct folio *folio;
vm_fault_t ret;
bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
!(vma->vm_flags & VM_SHARED);
+ int type, nr_pages, i;
+ unsigned long addr = vmf->address;
/* Did we COW the page? */
if (is_cow)
@@ -4780,24 +4783,44 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
return VM_FAULT_OOM;
}
+ folio = page_folio(page);
+ nr_pages = folio_nr_pages(folio);
+
+ if (unlikely(userfaultfd_armed(vma))) {
+ nr_pages = 1;
+ } else if (nr_pages > 1) {
+ unsigned long start = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
+ unsigned long end = start + nr_pages * PAGE_SIZE;
+
+ /* In case the folio size in page cache beyond the VMA limits. */
+ addr = max(start, vma->vm_start);
+ nr_pages = (min(end, vma->vm_end) - addr) >> PAGE_SHIFT;
+
+ page = folio_page(folio, (addr - start) >> PAGE_SHIFT);
I still don't really follow the logic in this else if block. Isn't it possible
that finish_fault() gets called with a page from a folio that isn't aligned with
vmf->address?
For example, let's say we have a file who's size is 64K and which is cached in a
single large folio in the page cache. But the file is mapped into a process at
VA 16K to 80K. Let's say we fault on the first page (VA=16K). You will calculate
For shmem, this doesn't happen because the VA is aligned with the hugepage size
in the shmem_get_unmapped_area() function. See patch 7.
Certainly agree that shmem can always make sure that it packs a vma in a way
such that its folios are naturally aligned in VA when faulting in memory. If you
mremap it, that alignment will be lost; I don't think that would be a problem
When mremap it, it will also call shmem_get_unmapped_area() to align the
VA, but for mremap() with MAP_FIXED flag as David pointed out, yes, this
patch may be not work perfectly.
for a single process; mremap will take care of moving the ptes correctly and
this path is not involved.
But what about the case when a process mmaps a shmem region, then forks, then
the child mremaps the shmem region. Then the parent faults in a THP into the
region (nicely aligned). Then the child faults in the same offset in the region
and gets the THP that the parent allocated; that THP will be aligned in the
parent's VM space but not in the child's.
Sorry, I did not get your point here. IIUC, the child's VA will also be
aligned if the child mremap is not set MAP_FIXED, since the child's
mremap will still call shmem_get_unmapped_area() to find an aligned new
VA. Please correct me if I missed your point.
start=0 and end=64K I think?
Yes. Unfortunately, some file systems that support large mappings do not perform
alignment for multi-size THP (non-PMD sized, for example: 64K). I think this
requires modification to __get_unmapped_area--->thp_get_unmapped_area_vmflags()
or file->f_op->get_unmapped_area() to align VA for multi-size THP in future.
By nature of the fact that a file mapping is shared between multiple processes
and each process can map it where ever it wants down to 1 page granularity, its
impossible for any THP containing a part of that file to be VA-aligned in every
process it is mapped in.
Yes, so let me re-polish this patch. Thanks.
So before adding that VA alignment changes, only allow building the large folio
mapping for anonymous shmem:
diff --git a/mm/memory.c b/mm/memory.c
index 936377220b77..9e4d51826d23 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4786,7 +4786,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
folio = page_folio(page);
nr_pages = folio_nr_pages(folio);
- if (unlikely(userfaultfd_armed(vma))) {
+ if (unlikely(userfaultfd_armed(vma)) || !vma_is_anon_shmem(vma)) {
If the above theoretical flow for fork & mremap is valid, then I don't think
this is sufficient.
nr_pages = 1;
} else if (nr_pages > 1) {
unsigned long start = ALIGN_DOWN(vmf->address, nr_pages *
PAGE_SIZE);
Additionally, I think this path will end up mapping the entire folio (as long as
it fits in the VMA). But this bypasses the fault-around configuration. As I
think I mentioned against the RFC, this will inflate the RSS of the process and
can cause behavioural changes as a result. I believe the current advice is to
disable fault-around to prevent this kind of bloat when needed.
With above change, I do not think this is a problem? since users already want to
use mTHP for anonymous shmem.
It might be that you need a special variant of finish_fault() for shmem?
+ }
vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
- vmf->address, &vmf->ptl);
+ addr, &vmf->ptl);
if (!vmf->pte)
return VM_FAULT_NOPAGE;
/* Re-check under ptl */
- if (likely(!vmf_pte_changed(vmf))) {
- struct folio *folio = page_folio(page);
- int type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
-
- set_pte_range(vmf, folio, page, 1, vmf->address);
- add_mm_counter(vma->vm_mm, type, 1);
- ret = 0;
- } else {
- update_mmu_tlb(vma, vmf->address, vmf->pte);
+ if (nr_pages == 1 && unlikely(vmf_pte_changed(vmf))) {
+ update_mmu_tlb(vma, addr, vmf->pte);
+ ret = VM_FAULT_NOPAGE;
+ goto unlock;
+ } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
+ for (i = 0; i < nr_pages; i++)
+ update_mmu_tlb(vma, addr + PAGE_SIZE * i, vmf->pte + i);
ret = VM_FAULT_NOPAGE;
+ goto unlock;
}
+ set_pte_range(vmf, folio, page, nr_pages, addr);
+ type = is_cow ? MM_ANONPAGES : mm_counter_file(folio);
+ add_mm_counter(vma->vm_mm, type, nr_pages);
+ ret = 0;
+
+unlock:
pte_unmap_unlock(vmf->pte, vmf->ptl);
return ret;
}
