Re: [PATCH] Use of getblk differs between locations

[Andrew Morton <akpm@osdl.org>]

Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz> 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...