lkml.org 
[lkml]   [2020]   [Sep]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/5] mm: Introduce mm_struct.has_pinned
On Sun, Sep 27, 2020 at 11:45:30AM -0700, Linus Torvalds wrote:
> On Sun, Sep 27, 2020 at 11:16 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Btw, I'm not convinced about the whole "turn the pte read-only and
> > then back". If the fork races with another thread doing a pinning
> > fast-GUP on another CPU, there are memory ordering issues etc too.
> > That's not necessarily visible on x86 (the "turn read-only being a
> > locked op will force serialization), but it all looks dodgy as heck.

Oh. Yes, looking again the atomics in the final arrangement of
copy_present_page() aren't going to be strong enough to order this.

Sorry for missing, wasn't able to look very deeply during the weekend.

Not seeing an obvious option besides adding a smp_mb() before
page_maybe_dma_pinned() as Peter once suggested.

> .. looking at it more, I also think it could possibly lose the dirty
> bit for the case where another CPU did a HW dirty/accessed bit update
> in between the original read of the pte, and then us writing back the
> writable pte again.

Ah, I see:

set_pte_at(src_mm, addr, src_pte, pte);

wants to be some arch specific single bit ptep_clear_wrprotect()..

Thanks,
Jason

\
 
 \ /
  Last update: 2020-09-28 14:50    [W:0.131 / U:0.068 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site