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SubjectRe: [PATCH] mm: avoid blocking lock_page() in kcompactd
On Thu, Feb 13, 2020 at 06:08:24PM +0100, Michal Hocko wrote:
> On Thu 13-02-20 08:46:07, Matthew Wilcox wrote:
> > On Thu, Feb 13, 2020 at 08:48:47AM +0100, Michal Hocko wrote:
> > > Can we pursue on this please? An explicit NOFS scope annotation with a
> > > reference to compaction potentially locking up on pages in the readahead
> > > would be a great start.
> >
> > How about this (on top of the current readahead series):
> >
> > diff --git a/mm/readahead.c b/mm/readahead.c
> > index 29ca25c8f01e..32fd32b913da 100644
> > --- a/mm/readahead.c
> > +++ b/mm/readahead.c
> > @@ -160,6 +160,16 @@ unsigned long page_cache_readahead_limit(struct address_space *mapping,
> > .nr_pages = 0,
> > };
> >
> > + /*
> > + * Partway through the readahead operation, we will have added
> > + * locked pages to the page cache, but will not yet have submitted
> > + * them for I/O. Adding another page may need to allocate
> > + * memory, which can trigger memory migration. Telling the VM
>
> I would go with s@migration@compaction@ because it would make it more
> obvious that this is about high order allocations.

Perhaps even just 'reclaim' -- it's about compaction today, but tomorrow's
VM might try to reclaim these pages too. They are on the LRU, after all.

So I currently have:

/*
* Partway through the readahead operation, we will have added
* locked pages to the page cache, but will not yet have submitted
* them for I/O. Adding another page may need to allocate memory,
* which can trigger memory reclaim. Telling the VM we're in
* the middle of a filesystem operation will cause it to not
* touch file-backed pages, preventing a deadlock. Most (all?)
* filesystems already specify __GFP_NOFS in their mapping's
* gfp_mask, but let's be explicit here.
*/

Thanks!

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