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

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Message-ID: <a6b8728d-7382-9316-412d-dd48b5e7c41a@intel.com>
Date:   Thu, 16 Apr 2020 08:36:50 -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 7:59 AM, Claudio Imbrenda wrote:
> 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.
...
>> 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

Ahh, so this is *just* intended to precede I/O done on the page, when a
non-host entity is touching the memory?

That seems inconsistent with the process_vm_readv/writev() paths which
set FOLL_PIN on their pin_remote_user_pages() requests, but don't do I/O
to the memory.