Re: [RFC 2/9] Use NOMEMALLOC reclaim to allow reclaim if PF_MEMALLOC is set

Christoph Lameter
2007-08-20T19:00:59Z

On Sat, 18 Aug 2007, Pavel Machek wrote:

> > The reclaim is of particular important to stacked filesystems that may
> > do a lot of allocations in the write path. Reclaim will be working
> > as long as there are clean file backed pages to reclaim.
>
> I don't get it. Lets say that we have stacked filesystem that needs
> it. That filesystem is broken today.
>
> Now you give it second chance by reclaiming clean pages, but there are
> no guarantees that we have any.... so that filesystem is still broken
> with your patch...?

There is a guarantee that we have some because the user space program is
executing. Meaning the executable pages can be retrieved. The amount
dirty memory in the system is limited by the dirty_ratio. So the VM can
only get into trouble if there is a sufficient amount of anonymous pages
and all executables have been reclaimed. That is pretty rare.

Plus the same issue can happen today. Writes are usually not completed
during reclaim. If the writes are sufficiently deferred then you have the
same issue now.