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

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Message-Id: <20200416165900.68bd4dba@p-imbrenda>
Date:   Thu, 16 Apr 2020 16:59:00 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Dave Hansen <dave.hansen@...el.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 Thu, 16 Apr 2020 07:20:48 -0700
Dave Hansen <dave.hansen@...el.com> wrote:

> 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?

it's in the fault handler for the exception the CPU will get when
attempting to write the data to the protected page