Re: [PATCH] Use of getblk differs between locations

From: Andrew Morton
Date: Tue Oct 11 2005 - 00:03:58 EST

Mikulas Patocka <mikulas@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >> I liked what linux-2.0 did in this case --- if the kernel was out of
> >> memory, getblk just took another buffer, wrote it if it was dirty and used
> >> it. Except for writeable loopback device (where writing one buffer
> >> generates more dirty buffers), it couldn't deadlock.
> >
> > Wouldn't it be better if bread() were to return ERR_PTR(-EIO) or
> > ERR_PTR(-ENOMEM)? Big change.
> No. Out of memory condition can happen even under normal circumstances
> under lightly loaded system. Think of a situation when dirty file-mapped
> pages fill up the whole memory, now a burst of packets from network comes
> that fills up kernel atomic reserve, you have zero pages free --- and what
> now? --- returning ENOMEM and dropping dirty pages without writing them is
> wrong, deadlocking (filesystem waits until memory manager frees some pages
> and memory manager waits until filesystem writes the dirty pages) is wrong
> too.

Well. The filesystem could just redirty the page and skip the writepage().
That's still deadlockable but I bet the kernel would recover in the vast
majority of cases.

Still, this is all very theoretical - there are no plans to make getblk
bail out on oom - AFAIK nobody has been able to demonstrate a testcase...
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