linux-kernel - Re: [PATCH 6.3.y] mm/hugetlb: revert use of page_cache_next

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Message-ID: <b5bd2b39-7e1e-148f-7462-9565773f6d41@oracle.com>
Date:   Wed, 7 Jun 2023 13:35:05 -0700
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     Greg KH <greg@...ah.com>
Cc:     stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        songmuchun@...edance.com, mike.kravetz@...cle.com,
        Ackerley Tng <ackerleytng@...gle.com>
Subject: Re: [PATCH 6.3.y] mm/hugetlb: revert use of page_cache_next_miss()

On 6/7/23 11:33 AM, Greg KH wrote:
> On Tue, Jun 06, 2023 at 11:13:05AM -0700, Sidhartha Kumar wrote:
>> 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@...r.kernel.org> #v6.3
>>>> Reported-by: Ackerley Tng <ackerleytng@...gle.com>
>>>> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@...cle.com>
>>>> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.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?
>>
>> 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
> 
> Great, is that going to Linus's tree soon?
> 

Andrew just added it to mm-hotfixes-stable so it should be in Linus's 
tree soon.

Thanks,
Sidhartha Kumar

> thanks,
> 
> greg k-h