linux-kernel - Re: [PATCH] mm/filemap: do not allocate cache pages beyond end of file at read

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Date:   Tue, 29 Oct 2019 17:25:27 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Johannes Weiner <hannes@...xchg.org>,
        Steven Whitehouse <swhiteho@...hat.com>
Subject: Re: [PATCH] mm/filemap: do not allocate cache pages beyond end of
 file at read

On 28/10/2019 15.57, Kirill A. Shutemov wrote:
> On Mon, Oct 28, 2019 at 01:47:16PM +0100, Linus Torvalds wrote:
>> On Mon, Oct 28, 2019 at 1:42 PM Kirill A. Shutemov <kirill@...temov.name> wrote:
>>>
>>> I've tried something of this sort back in 2013:
>>>
>>> http://lore.kernel.org/r/1377099441-2224-1-git-send-email-kirill.shutemov@linux.intel.com
>>>
>>> and I've got push back.
>>>
>>> Apparently, some filesystems may not have valid i_size before >readpage().
>>> Not sure if it's still the case...
>>
>> Well, I agree that there might be some network filesystem that might
>> have inode sizes that are stale, but if that's the case then I don't
>> think your previous patch works either.
>>
>> It too will avoid the readpage() if the read position is beyond i_size.
>>
>> No?
> 
> Yes. That's the reason the patch was rejected back then.
> 
> My point is that we need to make sure that this patch not break anything.
> 

I think all network filesystems which synchronize metadata lazily should be
marked. For example as "SB_VOLATILE". And vfs could handle them specially.

For this case generic_file_buffered_read() could call for them readpages
for single page (rather than readpage) to let filesystem revalidate
metadata and drop unneeded page without inserting it into inode and lru.