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

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Message-Id: <20200416183431.7216e1d1@p-imbrenda>
Date:   Thu, 16 Apr 2020 18:34:31 +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 08:36:50 -0700
Dave Hansen <dave.hansen@...el.com> wrote:

> 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?

yep

> 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.

FOLL_PIN simply indicates potential access to the content of the page,
not just for I/O.

so yes, we are overdoing arch_make_page_accessible() in some cases,
because we can't tell when a page will be used for I/O and when not.

In most cases this will boil down to checking a flag and doing nothing,
for example in case the page was already accessible.

Also note that making the page accessible because of a FOLL_PIN in
absence of I/O will probably later on spare us from triggering and
handling the exception that would have caused us to make the page
accessible anyway.