linux-kernel - Re: [PATCH v4 2/2] mm/gup/writeback: add callbacks for inaccessible pages

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Message-ID: <de56aa8e-9035-4b68-33cb-15682d073e26@intel.com>
Date:   Thu, 16 Apr 2020 07:20:48 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>, linux-next@...r.kernel.org,
        akpm@...ux-foundation.org, jack@...e.cz, kirill@...temov.name,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        borntraeger@...ibm.com, david@...hat.com, aarcange@...hat.com,
        linux-mm@...ck.org, frankja@...ux.ibm.com, sfr@...b.auug.org.au,
        jhubbard@...dia.com, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, Will Deacon <will@...nel.org>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH v4 2/2] mm/gup/writeback: add callbacks for inaccessible
 pages

On 4/16/20 5:15 AM, Claudio Imbrenda wrote:
>> I assumed that this was all anonymous-only so it's always dirty before
>> writeback starts.
> it could also be mmapped

Let's say you have a mmap()'d ramfs file.  Another process calls which
doesn't have it mapped calls sys_write() and writes to the file.

This means that host host has to write to the physical page and must do
arch_make_page_accessible() in the sys_write() path somewhere.

There is a get_user_pages() in that path, but it's on the _source_
buffer, not the ramfs page because the ramfs page is not mapped.
There's also no __test_set_page_writeback() because you can't write back
ramfs.

Where is the arch_make_page_accessible() in this case on the ramfs page?