Re: [PATCH mmotm] mm/swap: fix livelock in __read_swap_cache_async()

[Johannes Weiner]

On Thu, May 21, 2020 at 10:56:20PM -0700, Hugh Dickins wrote:
> I've only seen this livelock on one machine (repeatably, but not to
> order), and not fully analyzed it - two processes seen looping around
> getting -EEXIST from swapcache_prepare(), I guess a third (at lower
> priority? but wanting the same cpu as one of the loopers? preemption
> or cond_resched() not enough to let it back in?) set SWAP_HAS_CACHE,
> then went off into direct reclaim, scheduled away, and somehow could
> not get back to add the page to swap cache and let them all complete.
> 
> Restore the page allocation in __read_swap_cache_async() to before
> the swapcache_prepare() call: "mm: memcontrol: charge swapin pages
> on instantiation" moved it outside the loop, which indeed looks much
> nicer, but exposed this weakness.  We used to allocate new_page once
> and then keep it across all iterations of the loop: but I think that
> just optimizes for a rare case, and complicates the flow, so go with
> the new simpler structure, with allocate+free each time around (which
> is more considerate use of the memory too).
> 
> Fix the comment on the looping case, which has long been inaccurate:
> it's not a racing get_swap_page() that's the problem here.
> 
> Fix the add_to_swap_cache() and mem_cgroup_charge() error recovery:
> not swap_free(), but put_swap_page() to undo SWAP_HAS_CACHE, as was
> done before; but delete_from_swap_cache() already includes it.
> 
> And one more nit: I don't think it makes any difference in practice,
> but remove the "& GFP_KERNEL" mask from the mem_cgroup_charge() call:
> add_to_swap_cache() needs that, to convert gfp_mask from user and page
> cache allocation (e.g. highmem) to radix node allocation (lowmem), but
> we don't need or usually apply that mask when charging mem_cgroup.
> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

> Mostly fixing mm-memcontrol-charge-swapin-pages-on-instantiation.patch
> but now I see that mm-memcontrol-delete-unused-lrucare-handling.patch
> made a further change here (took an arg off the mem_cgroup_charge call):
> as is, this patch is diffed to go on top of both of them, and better
> that I get it out now for Johannes look at; but could be rediffed for
> folding into blah-instantiation.patch later.

IMO it's worth having as a separate change. Joonsoo was concerned
about the ordering but I didn't see it. Having this sequence of
changes on record would be good for later reference.