Messages in this thread | | | Subject | Re: [PATCH] mm, vmscan: do not loop on too_many_isolated for ever | From | Tetsuo Handa <> | Date | Thu, 20 Jul 2017 19:44:11 +0900 |
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Hugh Dickins wrote: > You probably won't welcome getting into alternatives at this late stage; > but after hacking around it one way or another because of its pointless > lockups, I lost patience with that too_many_isolated() loop a few months > back (on realizing the enormous number of pages that may be isolated via > migrate_pages(2)), and we've been running nicely since with something like: > > bool got_mutex = false; > > if (unlikely(too_many_isolated(pgdat, file, sc))) { > if (mutex_lock_killable(&pgdat->too_many_isolated)) > return SWAP_CLUSTER_MAX; > got_mutex = true; > } > ... > if (got_mutex) > mutex_unlock(&pgdat->too_many_isolated); > > Using a mutex to provide the intended throttling, without an infinite > loop or an arbitrary delay; and without having to worry (as we often did) > about whether those numbers in too_many_isolated() are really appropriate. > No premature OOMs complained of yet.
Roughly speaking, there is a moment where shrink_inactive_list() acts like below.
bool got_mutex = false;
if (!current_is_kswapd()) { if (mutex_lock_killable(&pgdat->too_many_isolated)) return SWAP_CLUSTER_MAX; got_mutex = true; }
// kswapd is blocked here waiting for !current_is_kswapd().
if (got_mutex) mutex_unlock(&pgdat->too_many_isolated);
> > But that was on a different kernel, and there I did have to make sure > that PF_MEMALLOC always prevented us from nesting: I'm not certain of > that in the current kernel (but do remember Johannes changing the memcg > end to make it use PF_MEMALLOC too). I offer the preview above, to see > if you're interested in that alternative: if you are, then I'll go ahead > and make it into an actual patch against v4.13-rc.
I don't know what your actual patch looks like, but the problem is that pgdat->too_many_isolated waits for kswapd while kswapd waits for pgdat->too_many_isolated; nobody can unlock pgdat->too_many_isolated if once we hit it.
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