On 28.04.24 21:01, Peter Xu wrote:
Commit a12083d721d7 added hugepd handling for gup-slow, reusing gup-fast
functions. follow_hugepd() correctly took the vma pointer in, however
didn't pass it over into the lower functions, which was overlooked.
The issue is gup_fast_hugepte() uses the vma pointer to make the correct
decision on whether an unshare is needed for a FOLL_PIN|FOLL_LONGTERM. Now
without vma ponter it will constantly return "true" (needs an unshare) for
a page cache, even though in the SHARED case it will be wrong to unshare.
The other problem is, even if an unshare is needed, it now returns 0 rather
than -EMLINK, which will not trigger a follow up FAULT_FLAG_UNSHARE fault.
That will need to be fixed too when the unshare is wanted.
gup_longterm test didn't expose this issue in the past because it didn't
yet test R/O unshare in this case, another separate patch will enable that
in future tests.
Fix it by passing vma correctly to the bottom, rename gup_fast_hugepte()
back to gup_hugepte() as it is shared between the fast/slow paths, and also
allow -EMLINK to be returned properly by gup_hugepte() even though gup-fast
will take it the same as zero.
Reported-by: David Hildenbrand <david@xxxxxxxxxx>
Fixes: a12083d721d7 ("mm/gup: handle hugepd for follow_page()")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
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LGTM
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
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Cheers,
David / dhildenb
