On Wed, Feb 26, 2025 at 10:17 AM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2025/2/26 19:48, Brian Geffon wrote:
> > When handling faults for anon shmem finish_fault() will attempt to install
> > ptes for the entire folio. Unfortunately if it encounters a single
> > non-pte_none entry in that range it will bail, even if the pte that
> > triggered the fault is still pte_none. When this situation happens the
> > fault will be retried endlessly never making forward progress.
> >
> > This patch fixes this behavior and if it detects that a pte in the range
> > is not pte_none it will fall back to setting just the pte for the
> > address that triggered the fault.
>
> Could you describe in detail how this situation occurs? How is the none
> pte inserted within the range of the large folio? Because we have checks
> in shmem to determine if a large folio is suitable.
We're seeing it because of racing shmem_undo_range() calls, basically
we have a portion of the folio is zapped and that prevents
finish_fault() from ever completing because it has an assumption that
the entire range must be pte_none.
>
> Anyway, if we find the pte_range_none() is false, we can fallback to
> per-page fault as the following code shows (untested), which seems more
> simple?
Yah, I like this approach. I'll send a v2 with you as a Suggested-by.
>
> diff --git a/mm/memory.c b/mm/memory.c
> index a8196ae72e9a..8a2a9fda5410 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5219,7 +5219,12 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
> !(vma->vm_flags & VM_SHARED);
> int type, nr_pages;
> - unsigned long addr = vmf->address;
> + unsigned long addr;
> + bool fallback_per_page = false;
> +
> +
> +fallback:
> + addr = vmf->address;
>
> /* Did we COW the page? */
> if (is_cow)
> @@ -5258,7 +5263,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> * approach also applies to non-anonymous-shmem faults to avoid
> * inflating the RSS of the process.
> */
> - if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))) {
> + if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))
> + || unlikely(fallback_per_page)) {
> nr_pages = 1;
> } else if (nr_pages > 1) {
> pgoff_t idx = folio_page_idx(folio, page);
> @@ -5294,9 +5300,9 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> ret = VM_FAULT_NOPAGE;
> goto unlock;
> } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> - update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
> - ret = VM_FAULT_NOPAGE;
> - goto unlock;
> + fallback_per_page = true;
> + pte_unmap_unlock(vmf->pte, vmf->ptl);
> + goto fallback;
> }
>
> folio_ref_add(folio, nr_pages - 1);
>
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Fixes: 43e027e41423 ("mm: memory: extend finish_fault() to support large folio")
> > Reported-by: Marek Maslanka <mmaslanka@xxxxxxxxxx>
> > Signed-off-by: Brian Geffon <bgeffon@xxxxxxxxxx>
> > ---
> > mm/memory.c | 19 ++++++++++++++++---
> > 1 file changed, 16 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index b4d3d4893267..32de626ec1da 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5258,9 +5258,22 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
> > ret = VM_FAULT_NOPAGE;
> > goto unlock;
> > } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> > - update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
> > - ret = VM_FAULT_NOPAGE;
> > - goto unlock;
> > + /*
> > + * We encountered a set pte, let's just try to install the
> > + * pte for the original fault if that pte is still pte none.
> > + */
> > + pgoff_t idx = (vmf->address - addr) / PAGE_SIZE;
> > +
> > + if (!pte_none(ptep_get_lockless(vmf->pte + idx))) {
> > + update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
> > + ret = VM_FAULT_NOPAGE;
> > + goto unlock;
> > + }
> > +
> > + vmf->pte = vmf->pte + idx;
> > + page = folio_page(folio, idx);
> > + addr = vmf->address;
> > + nr_pages = 1;
> > }
> >
> > folio_ref_add(folio, nr_pages - 1);
