Re: [RFC] per thread page reservation patch

[Nikita Danilov]

Andrew Morton <akpm@xxxxxxxx> writes:

> Nikita Danilov <nikita@xxxxxxxxxxxxx> wrote:
>>
>> > And the whole idea is pretty flaky really - how can one precalculate how
>> > much memory an arbitrary md-on-dm-on-loop-on-md-on-NBD stack will want to
>> > use?  It really would be better if we could drop the whole patch and make
>> > reiser4 behave more sanely when its writepage is called with for_reclaim=1.
>> 
>> Reiser4 doesn't use this for ->writepage(), by the way. This is used by
>> tree balancing code to assure that balancing cannot get -ENOMEM in the
>> middle of tree modification, because undo is _so_ very complicated.
>
> Oh.  And that involves performing I/O, yes?

Yes, balancing may read tree or bitmap nodes from the disk.

>
> Why does the filesystem risk going oom during the rebalance anyway?  Is it
> doing atomic allocations?

No, just __alloc_pages(GFP_KERNEL, 0, ...) returns NULL. When this
happens, the only thing balancing can do is to panic.

Nikita.


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