linux-kernel - Re: [PATCH v7 06/11] filemap: cap PTE range to be created to allowed zero fill in folio_map

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Date: Thu, 13 Jun 2024 10:16:10 +0200
From: David Hildenbrand <david@...hat.com>
To: Luis Chamberlain <mcgrof@...nel.org>
Cc: Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>,
 yang@...amperecomputing.com, linmiaohe@...wei.com, muchun.song@...ux.dev,
 osalvador@...e.de, "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
 david@...morbit.com, djwong@...nel.org, chandan.babu@...cle.com,
 brauner@...nel.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
 hare@...e.de, linux-kernel@...r.kernel.org, Zi Yan <zi.yan@...t.com>,
 linux-xfs@...r.kernel.org, p.raghav@...sung.com,
 linux-fsdevel@...r.kernel.org, hch@....de, gost.dev@...sung.com,
 cl@...amperecomputing.com, john.g.garry@...cle.com
Subject: Re: [PATCH v7 06/11] filemap: cap PTE range to be created to allowed
 zero fill in folio_map_range()

On 13.06.24 10:13, Luis Chamberlain wrote:
> On Thu, Jun 13, 2024 at 10:07:15AM +0200, David Hildenbrand wrote:
>> On 13.06.24 09:57, Luis Chamberlain wrote:
>>> On Wed, Jun 12, 2024 at 08:08:15PM +0100, Matthew Wilcox wrote:
>>>> On Fri, Jun 07, 2024 at 02:58:57PM +0000, Pankaj Raghav (Samsung) wrote:
>>>>> From: Pankaj Raghav <p.raghav@...sung.com>
>>>>>
>>>>> Usually the page cache does not extend beyond the size of the inode,
>>>>> therefore, no PTEs are created for folios that extend beyond the size.
>>>>>
>>>>> But with LBS support, we might extend page cache beyond the size of the
>>>>> inode as we need to guarantee folios of minimum order. Cap the PTE range
>>>>> to be created for the page cache up to the max allowed zero-fill file
>>>>> end, which is aligned to the PAGE_SIZE.
>>>>
>>>> I think this is slightly misleading because we might well zero-fill
>>>> to the end of the folio.  The issue is that we're supposed to SIGBUS
>>>> if userspace accesses pages which lie entirely beyond the end of this
>>>> file.  Can you rephrase this?
>>>>
>>>> (from mmap(2))
>>>>          SIGBUS Attempted access to a page of the buffer that lies beyond the end
>>>>                 of the mapped file.  For an explanation of the treatment  of  the
>>>>                 bytes  in  the  page that corresponds to the end of a mapped file
>>>>                 that is not a multiple of the page size, see NOTES.
>>>>
>>>>
>>>> The code is good though.
>>>>
>>>> Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>
>>>
>>> Since I've been curating the respective fstests test to test for this
>>> POSIX corner case [0] I wanted to enable the test for tmpfs instead of
>>> skipping it as I originally had it, and that meant also realizing mmap(2)
>>> specifically says this now:
>>>
>>> Huge page (Huge TLB) mappings
>>
>> Confusion alert: this likely talks about hugetlb (MAP_HUGETLB), not THP and
>> friends.
>>
>> So it might not be required for below changes.
> 
> Thanks, I had to ask as we're dusting off this little obscure corner of
> the universe. Reason I ask, is the test fails for tmpfs with huge pages,
> and this patch fixes it, but it got me wondering the above applies also
> to tmpfs with huge pages.

Is it tmpfs with THP/large folios or shmem with hugetlb? I assume the 
tmpfs with THP. There are not really mmap/munmap restrictions to THP and 
friends (because it's supposed to be "transparent" :) ).

-- 
Cheers,

David / dhildenb