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SubjectRe: [PATCH] mm: moving dirty pages balancing to pdfludh entirely
Date
Ananiev, Leonid I writes:
> Nikita Danilov wtites:
> > performs page-out even if queue is congested.
> Yes. If user thread which generates dirty pages need in
> reclaimed memory it consider own dirty page as candidate for page-out.
> It functions as before patching.
>
> > Intent of this is to throttle writers.
> I suppose you means dirtier or write(2) caller but not writepage()
> caller. The dirtier is throttled with backing_dev_info logic as before
> patching.

I meant ->writepages() used by balance_dirty_pages(), see below.

>
> While pdflush thread sorts pages for page-out it does not
> consider as a candidate a page to be written with congested queue.
> Pdflush thread functions as before patching. Pdflush tends to make pages
> un-dirty without overload memory or IO and it is not need to let pdflush

This assumption is valid for ext2, where ->writepages() simply sends
pages to the storage, but other file systems (like reiser4) do a *lot*
of work in ->writepages() path, allocating quite an amount of memory
before starting write-out. With your patch, this work is done from
pdflush, and won't be throttled by may_write_to_queue() check, thus
increasing a risk of allocation failure.

> do page-out with congested queue as you have proposed.
>
> Leonid

Nikita.
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