lkml.org 
[lkml]   [2003]   [May]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Race between vmtruncate and mapped areas?
On Wed, May 14, 2003 at 11:53:19AM -0700, Andrew Morton wrote:
> converting i_sem to an rwsem and taking it in the pagefault would certainly
> stitch it up. Unpopular, very messy.

and very slow, down_read on every page fault wouldn't scale

> Could "truncate file" return some code to say pages were left behind, so
> truncate re-runs zap_page_range()? Sounds unpleasant.
>
>
> Yes, re-checking the page against i_size from do_no_page() would fix it up.

think if there are two truncates, one zapping the entere file, and
another restoring the previous i_size, in such case the new_page will be
wrong, as it won't be zeroed out. I mean if we do anything about it, we
should close all races and make it 100% correct.

My fix has no scalability cost, no indirect calls, touches mostly just
hot cachelines anyways, and addresses the multiple truncate case too.


> But damn, that's another indirect call, 64-bit math, etc on _every_
> file-backed pagefault.
>
>
> Remind me again what problem this whole thing is currently causing?

the only thing I can imagine is an app trapping SIGBUS to garbage
collect the end of a file. So for example you truncate the file and you
wait the SIGBUS to know you've to re-extend it. it would be a legitimate
use, and this is a bug, it's not read against write that has no way to
synchronize anyways, however I doubt any application is being bitten by
this race.

Andrea
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:35    [W:0.746 / U:0.740 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site