lkml.org 
[lkml]   [2021]   [Apr]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC PATCH 09/37] mm: add per-mm mmap sequence counter for speculative page fault handling.
On Tue, Apr 06, 2021 at 06:44:34PM -0700, Michel Lespinasse wrote:
> The counter's write side is hooked into the existing mmap locking API:
> mmap_write_lock() increments the counter to the next (odd) value, and
> mmap_write_unlock() increments it again to the next (even) value.
>
> The counter's speculative read side is supposed to be used as follows:
>
> seq = mmap_seq_read_start(mm);
> if (seq & 1)
> goto fail;
> .... speculative handling here ....
> if (!mmap_seq_read_check(mm, seq)
> goto fail;
>
> This API guarantees that, if none of the "fail" tests abort
> speculative execution, the speculative code section did not run
> concurrently with any mmap writer.

So this is obviously safe, but it's also super excessive. Any change,
anywhere, will invalidate and abort a SPF.

Since you make a complete copy of the vma, you could memcmp it in its
entirety instead of this.

\
 
 \ /
  Last update: 2021-04-07 16:51    [W:0.174 / U:0.496 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site