Re: [PATCH 1/7] mm/thp: fix __split_huge_pmd_locked() on shmem migration entry

[Kirill A. Shutemov]

On Tue, Jun 01, 2021 at 02:05:45PM -0700, Hugh Dickins wrote:
> Stressing huge tmpfs page migration racing hole punch often crashed on the
> VM_BUG_ON(!pmd_present) in pmdp_huge_clear_flush(), with DEBUG_VM=y kernel;
> or shortly afterwards, on a bad dereference in __split_huge_pmd_locked()
> when DEBUG_VM=n.  They forgot to allow for pmd migration entries in the
> non-anonymous case.
> 
> Full disclosure: those particular experiments were on a kernel with more
> relaxed mmap_lock and i_mmap_rwsem locking, and were not repeated on the
> vanilla kernel: it is conceivable that stricter locking happens to avoid
> those cases, or makes them less likely; but __split_huge_pmd_locked()
> already allowed for pmd migration entries when handling anonymous THPs,
> so this commit brings the shmem and file THP handling into line.
> 
> Are there more places that need to be careful about pmd migration entries?
> None hit in practice, but several of those is_huge_zero_pmd() tests were
> done without checking pmd_present() first: I believe a pmd migration entry
> could end up satisfying that test.  Ah, the inversion of swap offset, to
> protect against L1TF, makes that impossible on x86; but other arches need
> the pmd_present() check, and even x86 ought not to apply pmd_page() to a
> swap-like pmd.  Fix those instances; __split_huge_pmd_locked() was not
> wrong to be checking with pmd_trans_huge() instead, but I think it's
> clearer to use pmd_present() in each instance.
> 
> And while there: make it clearer to the eye that the !vma_is_anonymous()
> and is_huge_zero_pmd() blocks make early returns (and don't return void).
> 
> Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Looks like a two fixes got squashed into one patch. Zero-page fix and
migration entries in __split_huge_pmd_locked() deserve separate patches,
no?

Maybe add VM_BUG_ON(!pmd_present()) in is_huge_zero_pmd()?

Also I wounder how much code we can remove if we would not establish
migration ptes for file pages. We can make these page table entries 'none'
on migration.

-- 
 Kirill A. Shutemov