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Date:   Tue, 6 Jun 2023 19:38:27 +0200
From:   Greg KH <greg@kroah.com>
To:     Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc:     stable@vger.kernel.org, linux-kernel@vger.kernel.org,
        songmuchun@bytedance.com, mike.kravetz@oracle.com,
        Ackerley Tng <ackerleytng@google.com>
Subject: Re: [PATCH 6.3.y] mm/hugetlb: revert use of page_cache_next_miss()
Message-ID: <2023060650-overlying-skiing-191d@gregkh>
References: <20230606172022.128441-1-sidhartha.kumar@oracle.com>
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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@vger.kernel.org> #v6.3
> Reported-by: Ackerley Tng <ackerleytng@google.com>
> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
> [1] https://lore.kernel.org/linux-mm/cover.1683069252.git.ackerleytng@google.com/
> ---
> 
> 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?

thanks,

greg k-h