lkml.org 
[lkml]   [2022]   [Mar]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2 13/15] mm: support GUP-triggered unsharing of anonymous pages
On Mon, Mar 21, 2022 at 05:15:06PM +0100, David Hildenbrand wrote:
> On 19.03.22 00:30, Jason Gunthorpe wrote:
> > On Tue, Mar 15, 2022 at 11:47:39AM +0100, David Hildenbrand wrote:
> >> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
> >> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
> >> on the page table entry will end up replacing the mapped anonymous page
> >> due to COW, resulting in the GUP pin no longer being consistent with the
> >> page actually mapped into the page table.
> >>
> >> The possible ways to deal with this situation are:
> >> (1) Ignore and pin -- what we do right now.
> >> (2) Fail to pin -- which would be rather surprising to callers and
> >> could break user space.
> >> (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
> >> pins.
> >>
> >> We want to implement 3) because it provides the clearest semantics and
> >> allows for checking in unpin_user_pages() and friends for possible BUGs:
> >> when trying to unpin a page that's no longer exclusive, clearly
> >> something went very wrong and might result in memory corruptions that
> >> might be hard to debug. So we better have a nice way to spot such
> >> issues.
> >>
> >> To implement 3), we need a way for GUP to trigger unsharing:
> >> FAULT_FLAG_UNSHARE. FAULT_FLAG_UNSHARE is only applicable to R/O mapped
> >> anonymous pages and resembles COW logic during a write fault. However, in
> >> contrast to a write fault, GUP-triggered unsharing will, for example, still
> >> maintain the write protection.
> >
> > Given the way this series has developed you might want to call this
> > FAULT_FLAG_MAKE_ANON_EXCLUSIVE
> >
> > Which strikes me as more directly connected to what it is trying to
> > do.
>
> I thought about something similar along those lines, and I think it
> would apply even when extending that mechanism to anything !anon inside
> a MAP_PRIVATE mapping.
>
> The whole
>
> const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;

I think the extra words are worthwhile, share makes me think about
MAP_SHARED as we don't really use shared anywhere else FWICT..

Jason

\
 
 \ /
  Last update: 2022-03-21 17:21    [W:0.392 / U:0.196 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site