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    SubjectRe: [patch 10/21] buffer heads: Support slab defrag
    On Tue, May 20, 2008 at 04:28:16PM -0700, Andrew Morton ( wrote:
    > It's more than efficiency. There are lots and lots of things we cannot
    > do in direct-reclaim context.
    > a) Can't lock pages (well we kinda sorta could, but generally code
    > will just trylock)
    > b) Cannot rely on the inode or the address_space being present in
    > memory after we have unlocked the page.
    > c) Cannot run iput(). Or at least, we couldn't five or six years
    > ago. afaik nobody has investigated whether the situation is now
    > better or worse.
    > d) lots of deadlock scenarios - need to test __GFP_FS basically everywhere
    > in which you share code with normal writeback paths.
    > Plus e), f), g) and h). Direct-reclaim is a hostile environment.
    > Things like b) are a real killer - nasty, subtle, rare,
    > memory-pressure-dependent crashes.

    Which basically means we can not do direct writeback at reclaim time?..

    Evgeniy Polyakov

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