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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Message-ID: <818f69fa-9dc7-4ca0-b3ab-a667cd1fb16d@redhat.com>
Date: Thu, 13 Jun 2024 10:07:15 +0200
From: David Hildenbrand <david@...hat.com>
To: Luis Chamberlain <mcgrof@...nel.org>, Matthew Wilcox
 <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>,
 yang@...amperecomputing.com, linmiaohe@...wei.com, muchun.song@...ux.dev,
 osalvador@...e.de
Cc: "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 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.

> ...
>         For mmap(), offset must be a multiple of the underlying huge page
>         size. The system automatically aligns length to be a multiple of
>         the underlying huge page size.
> 
> So do we need to adjust this patch with this:
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index ea78963f0956..9c8897ba90ff 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3617,6 +3617,7 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
>   	vm_fault_t ret = 0;
>   	unsigned long rss = 0;
>   	unsigned int nr_pages = 0, mmap_miss = 0, mmap_miss_saved, folio_type;
> +	unsigned int align = PAGE_SIZE;
>   
>   	rcu_read_lock();
>   	folio = next_uptodate_folio(&xas, mapping, end_pgoff);
> @@ -3636,7 +3637,10 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
>   		goto out;
>   	}
>   
> -	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
> +	if (folio_test_pmd_mappable(folio))
> +		align = 1 << folio_order(folio);
> +
> +	file_end = DIV_ROUND_UP(i_size_read(mapping->host), align) - 1;
>   	if (end_pgoff > file_end)
>   		end_pgoff = file_end;
> 
> [0] https://lore.kernel.org/all/20240611030203.1719072-3-mcgrof@kernel.org/
> 
>    Luis
> 

-- 
Cheers,

David / dhildenb