linux-kernel - Re: [PATCH RFC] fs/fcntl: add fcntl F_GET

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Date:   Mon, 28 Oct 2019 14:20:51 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Roman Gushchin <guro@...com>
Subject: Re: [PATCH RFC] fs/fcntl: add fcntl F_GET_RSS

On 28/10/2019 14.10, Matthew Wilcox wrote:
> On Mon, Oct 28, 2019 at 01:28:09PM +0300, Konstantin Khlebnikov wrote:
>> +	if (dax_mapping(mapping))
>> +		pages = READ_ONCE(mapping->nrexceptional);
>> +	else
>> +		pages = READ_ONCE(mapping->nrpages);
> 
> I'm not sure this is the right calculation for DAX files.  We haven't
> allocated any memory for DAX; we're just accessing storage directly.
> The entries in the page caache are just translation from file offset to
> physical address.
> 

Yep, makes sense. If RSS declared as memory usage then this chunk must do
pages = READ_ONCE(mapping->nrpages) unconditionally and report 0 for DAX.