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

[Claudio Imbrenda]

On Thu, 16 Apr 2020 08:36:50 -0700
Dave Hansen <dave.hansen@xxxxxxxxx> wrote:

> On 4/16/20 7:59 AM, Claudio Imbrenda wrote:
> > On Thu, 16 Apr 2020 07:20:48 -0700
> > Dave Hansen <dave.hansen@xxxxxxxxx> 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.