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    SubjectRe: POHMELFS high performance network filesystem. Transactions, failover, performance.
    On Wed, 14 May 2008 11:40:30 +0400 Evgeniy Polyakov <johnpol@2ka.mipt.ru> wrote:

    > Hi Andrew.
    >
    > On Tue, May 13, 2008 at 11:33:41PM -0700, Andrew Morton (akpm@linux-foundation.org) wrote:
    > > If any thread takes more than one kmap() at a time, it is deadlockable.
    > > Because there is a finite pool of kmaps. Everyone can end up holding
    > > one or more kmaps, then waiting for someone else to release one.
    >
    > It never takes the whole LAST_PKMAP maps. So the same can be applied to
    > any user who kmaps at least one page - while user waits for free slot,
    > it can be reused by someone else and so on.
    >
    > But it can be speed issue, on 32 bit machine with 8gb of ram essentially
    > all pages were highmem and required mapping, so this does slows things
    > down (probably a lot), so I will extend writeback path of the POHMELFS
    > not to kmap pages, but instead use ->sendpage(), which if needed will
    > map page one-by-one. Current approach when page is mapped and then
    > copied looks really beter since the only one sending function is used
    > which takes lock only single time.

    OK.

    > > Duplicating page_waitqueue() is bad. Exporting it is probably bad too.
    > > Better would be to help us work out why the core kernel infrastructure is
    > > unsuitable, then make it suitable.
    >
    > When ->writepage() is used, it has to wait until page is written (remote
    > side sent acknowledge), so if multiple pages are being written
    > simultaneously we either have to allocate shared structure or use
    > per-page wait.

    That sounds exactly like wait_on_page_writeback()?

    > Right now there are transactions (and they will be used
    > for all operations eventually), so this waiting can go away.
    > It is exactly the same logic which lock_page() uses.
    >
    > Will lock_page_killable()/__lock_page_killable() be exported to modules?

    Maybe, if there's a need. I see no particular problem with that.


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