Re: [PATCH 29/31] mm/memory: handle_pte_fault() use pte_offset_map_nolock()

[Hugh Dickins <hughd@google.com>]

On Mon, 22 May 2023, Qi Zheng wrote:
> On 2023/5/22 13:26, Hugh Dickins wrote:
> > handle_pte_fault() use pte_offset_map_nolock() to get the vmf.ptl which
> > corresponds to vmf.pte, instead of pte_lockptr() being used later, when
> > there's a chance that the pmd entry might have changed, perhaps to none,
> > or to a huge pmd, with no split ptlock in its struct page.
> > 
> > Remove its pmd_devmap_trans_unstable() call: pte_offset_map_nolock()
> > will handle that case by failing.  Update the "morph" comment above,
> > looking forward to when shmem or file collapse to THP may not take
> > mmap_lock for write (or not at all).
> > 
> > do_numa_page() use the vmf->ptl from handle_pte_fault() at first, but
> > refresh it when refreshing vmf->pte.
> > 
> > do_swap_page()'s pte_unmap_same() (the thing that takes ptl to verify a
> > two-part PAE orig_pte) use the vmf->ptl from handle_pte_fault() too; but
> > do_swap_page() is also used by anon THP's __collapse_huge_page_swapin(),
> > so adjust that to set vmf->ptl by pte_offset_map_nolock().
> > 
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> >   mm/khugepaged.c |  6 ++++--
> >   mm/memory.c     | 38 +++++++++++++-------------------------
> >   2 files changed, 17 insertions(+), 27 deletions(-)
> > 
...
> > diff --git a/mm/memory.c b/mm/memory.c
> > index c7b920291a72..4ec46eecefd3 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
...
> > @@ -4897,27 +4897,16 @@ static vm_fault_t handle_pte_fault(struct vm_fault
> > *vmf)
> >     vmf->pte = NULL;
> >     vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
> >   	} else {
> > -		/*
> > -		 * If a huge pmd materialized under us just retry later.  Use
> > -		 * pmd_trans_unstable() via pmd_devmap_trans_unstable()
> > instead
> > -		 * of pmd_trans_huge() to ensure the pmd didn't become
> > -		 * pmd_trans_huge under us and then back to pmd_none, as a
> > -		 * result of MADV_DONTNEED running immediately after a huge
> > pmd
> > -		 * fault in a different thread of this mm, in turn leading to
> > a
> > -		 * misleading pmd_trans_huge() retval. All we have to ensure
> > is
> > -		 * that it is a regular pmd that we can walk with
> > -		 * pte_offset_map() and we can do that through an atomic read
> > -		 * in C, which is what pmd_trans_unstable() provides.
> > -		 */
> > -		if (pmd_devmap_trans_unstable(vmf->pmd))
> > -			return 0;
> >     /*
> >   		 * A regular pmd is established and it can't morph into a huge
> > -		 * pmd from under us anymore at this point because we hold the
> > -		 * mmap_lock read mode and khugepaged takes it in write mode.
> > -		 * So now it's safe to run pte_offset_map().
> > +		 * pmd by anon khugepaged, since that takes mmap_lock in write
> > +		 * mode; but shmem or file collapse to THP could still morph
> > +		 * it into a huge pmd: just retry later if so.
> >   		 */
> > -		vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
> > +		vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
> > +						 vmf->address, &vmf->ptl);
> > +		if (unlikely(!vmf->pte))
> > +			return 0;
> 
> Just jump to the retry label below?

Shrug.  Could do.  But again I saw no reason to optimize this path,
the pmd_devmap_trans_unstable() treatment sets a good enough example.

Hugh