Re: forcing write-back from FS - again

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 21 Oct 2007 23:19:41 +0300 Artem Bityutskiy <Artem.Bityutskiy@nokia.com> wrote:

> Hi Andrew,
> 
> some time ago we were talking about doing write-back from inside a file-system 
> (http://marc.info/?l=linux-kernel&m=119097117713616&w=2). You said that I'm not 
> the only person who needs this, because the same thing is needed for delayed 
> allocation.
> 
> The problem is that if we initiate write-back from prepare_write() and we are 
> having a dirty page lock, we deadlock in write_cache_pages() which tries to 
> lock the same page.
> 
> You suggested to enhance struct writeback_control and put page that should be 
> skipped.
> 
> ...
>
> but it does not dot actually work, because if we have two processes forcing 
> write-back from write_page(), they will mutually deadlock (A waits in 
> write_cache_pages() on a page B has locked, B waits on inode or page A has locked).

Yeah, I was just thinking that as I read this ;)
 
> So this way is not ok, do you have any other ideas?
> 
> We could mark page clean temporarily before doing write-back, and mark it dirty 
> again, but this seems to be inefficient (although I'm not sure, need to dig 
> these functions deeper, but they _seem_ to traverse the radix tree and change 
> tags, so marking one page dirty may need to change many tags, but again, I did 
> not really dig tis yet).
> 
> I'd appreciate any suggestions. Thanks!

We could just skip locked pages altogether in writeback.  Perhaps in
WB_SYNC_NONE mode, or perhaps add a new flag in writeback_control to select
this behaviour.

It _should_ be the case that the number of locked-and-dirty pages which
writeback encounters is very small, so skipping locked pages during
writeback-for-memory-flushing won't have any significant effect.  The first
step should be to add a new /proc/vmstat field to count these pages and
then do broad testing (especially on blocksize<pagesize filesystems) to
confirm the theory.

We'll still need to synchronously lock the page in
writeback-for-data-integrity mode though.