Re: [PATCH 6.3.y] mm/hugetlb: revert use of page_cache_next_miss()

[Sidhartha Kumar]

On 6/6/23 10:38 AM, Greg KH wrote:
> On Tue, Jun 06, 2023 at 10:20:22AM -0700, Sidhartha Kumar wrote:
> > As reported by Ackerley[1], the use of page_cache_next_miss() in
> > hugetlbfs_fallocate() introduces a bug where a second fallocate() call to
> > same offset fails with -EEXIST. Revert this change and go back to the
> > previous method of using get from the page cache and then dropping the
> > reference on success.
> >
> > hugetlbfs_pagecache_present() was also refactored to use
> > page_cache_next_miss(), revert the usage there as well.
> >
> > User visible impacts include hugetlb fallocate incorrectly returning
> > EEXIST if pages are already present in the file. In addition, hugetlb
> > pages will not be included in core dumps if they need to be brought in via
> > GUP. userfaultfd UFFDIO_COPY also uses this code and will not notice pages
> > already present in the cache. It may try to allocate a new page and
> > potentially return ENOMEM as opposed to EEXIST.
> >
> > Fixes: d0ce0e47b323 ("mm/hugetlb: convert hugetlb fault paths to use alloc_hugetlb_folio()")
> > Cc: <stable@xxxxxxxxxxxxxxx> #v6.3
> > Reported-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> > Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
> > Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> >
> > [1] https://lore.kernel.org/linux-mm/cover.1683069252.git.ackerleytng@xxxxxxxxxx/
> > ---
> >
> > This revert is the safest way to fix 6.3. The upstream fix will either
> > fix page_cache_next_miss() itself or use Ackerley's patch to introduce a
> > new function to check if a page is present in the page cache. Both
> > directions are currently under review so we can use this safe and simple
> > fix for 6.3
>
> Is there any specific reason why we don't just wait for the fix for
> Linus's tree before applying this one, or applying the real fix instead?

I missed Andrew's message stating he would prefer the real fix[1].

Sorry for the noise,
Sidhartha Kumar

[1] 
https://lore.kernel.org/lkml/20230603022209.GA114055@monkey/T/#mea6c8a015dbea5f9c2be88b9791996f4be6c2de8
> thanks,
>
> greg k-h