Re: [Lsf-pc] [ATTEND] LSF/MM conference

[Jan Kara <jack@suse.cz>]

  Hello,

On Mon 06-02-12 00:14:58, Michel Lespinasse wrote:
> I would like to attend the LSF/MM summit in April this year. I do not
> have any formal topic proposals at this point; however there are
> several MM areas I am interested in:
> 
> - mmap_sem locking: I have done some work in the past to reduce
> mmap_sem hold times when page faults wait for transfering file pages
> from disk, as well as during large mlock operations. However mmap_sem
> can still be held for long times today when write page faults trigger
> dirty write throttling, or when the system is under memory pressure
> and page allocations within the page fault handler hit the ttfp path
> (I have some pending work in these areas that I'd like to submit
> shortly). This is an area that hasn't been much invested in, probably
> because the fact that most users only need a read lock suffices to
> mask the issues in many cases. However I expect it to become more
> important as we keep improving performance isolation between
> processes. One way we frequently hit mmap_sem related issues at Google
> is when building monitoring mechanisms that are expected to stay
> responsive when the monitored systems get into bad memory pressure
> situations.
  I'd be interested in this. Holding mmap_sem during write page faults
(->page_mkwrite) is causing problems with lock ordering when handling
filesystem freezing. I hope I can solve the problems by dropping mmap_sem
when we see filesystem is frozen, wait for it to thaw, retake mmap_sem and
return VM_FAULT_RETRY but I'd be happy for a less hacky solution...

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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