Re: [RFC PATCH] mm: retry writepages() on ENOMEM when doing an data integrity writeback

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2017/03/15 22:03, Michal Hocko wrote:
> On Wed 15-03-17 01:07:43, Theodore Ts'o wrote:
>> Unfortunately, this can indeed cause livelocks, since inside the
>> writepages() call, the file system is holding various mutexes, and
>> these mutexes may prevent the OOM killer from killing its targetted
>> victim if it is also holding on to those mutexes.
> 
> The victim might be looping inside do_writepages now instead (especially
> when the memory reserves are depleted), though. On the other hand the
> recent OOM killer changes do not rely on the oom victim exiting anymore.

True only if CONFIG_MMU=y.

> We try to reap as much memory from its address space as possible
> which alone should help us to move on. Even if that is not sufficient we
> will move on to another victim. So unless everything is in this path and
> all the memory is sitting unreachable from the reapable address space we
> should be safe.

If the caller is doing sync() or umount() syscall, isn't it reasonable
to bail out if fatal_signal_pending() is true because it is caller's
responsibility to check whether sync() or umount() succeeded? Though,
I don't know whether writepages() can preserve data for later retry by
other callers.

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