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Subject: Re: [PATCH RFC] fs/fcntl: add fcntl F_GET_RSS
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        Michal Hocko <mhocko@suse.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>
References: <157225848971.557.16257813537984792761.stgit@buzz>
 <20191028111034.GS2963@bombadil.infradead.org>
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Message-ID: <33978ec2-ac27-1b8b-ba33-3bd2c66aa016@yandex-team.ru>
Date:   Mon, 28 Oct 2019 14:20:51 +0300
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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.