Re: [PATCH] mm/lruvec: preemptively free dead folios during lru_add drain

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Apr 24, 2026 at 08:38:06AM -0700, JP Kobryn (Meta) wrote:
> On 4/23/26 4:53 PM, Barry Song wrote:
> > On Fri, Apr 24, 2026 at 7:46 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > > 
> > > On Fri, Apr 24, 2026 at 07:22:30AM +0800, Barry Song wrote:
> > > > On Fri, Apr 24, 2026 at 12:43 AM JP Kobryn (Meta) <jp.kobryn@linux.dev> wrote:
> > > > > 
> > > > > Of all observable lruvec lock contention in our fleet, we find that ~24%
> > > > > occurs when dead folios are present in lru_add batches at drain time. This
> > > > > is wasteful in the sense that the folio is added to the LRU just to be
> > > > > immediately removed via folios_put_refs(), incurring two unnecessary lock
> > > > > acquisitions.
> > > > > 
> > > > > Eliminate this overhead by preemptively cleaning up dead folios before they
> > > > > make it into the LRU. Use folio_ref_freeze() to filter folios whose only
> > > > > remaining refcount is the batch ref. When dead folios are found, move them
> > > > > off the add batch and onto a temporary batch to be freed.
> > > > > 
> > > > > During A/B testing on one of our prod instagram workloads (high-frequency
> > > > > short-lived requests), the patch intercepted almost all dead folios before
> > > > > they entered the LRU. Data collected using the mm_lru_insertion tracepoint
> > > > > shows the effectiveness of the patch:
> > > > > 
> > > > > Per-host LRU add averages at 95% CPU load
> > > > > (60 hosts each side, 3 x 60s intervals)
> > > > > 
> > > > >              dead folios/min  total folios/min   dead %
> > > > > unpatched:        1,297,785        19,341,986  6.7097%
> > > > > patched:                 14        19,039,996  0.0001%
> > > > > 
> > > > > Within this workload, we save ~2.6M lock acquisitions per minute per host
> > > > > as a result.
> > > > > 
> > > > > System-wide memory stats improved on the patched side also at 95% CPU load:
> > > > >   - direct reclaim scanning reduced 7%
> > > > >   - allocation stalls reduced 5.2%
> > > > >   - compaction stalls reduced 12.3%
> > > > >   - page frees reduced 4.9%
> > > > > 
> > > > > No regressions were observed in requests served per second or request tail
> > > > > latency (p99). Both metrics showed directional improvement at higher CPU
> > > > > utilization (comparing 85% to 95%).
> > > > > 
> > > > > Signed-off-by: JP Kobryn (Meta) <jp.kobryn@linux.dev>
> > > > 
> > > > Hi JP,
> > > > I’m seeing a large number of "BAD page" bugs.
> > > > Not sure if it’s related, but reverting this patch
> > > > seems to fix the issue.
> > > > 
> > > > [ 2869.365978] BUG: Bad page state in process uname  pfn:3a5417
> > > > [ 2869.365981] page: refcount:0 mapcount:0 mapping:0000000000000000
> > > > index:0x724884c20 pfn:0x3a5417
> > > > [ 2869.365983] flags:
> > > > 0x17ffffc0020908(uptodate|active|owner_2|swapbacked|node=0|zone=2|lastcpupid=0x1fffff)
> > > 
> > > Hi Barry, are you using MGLRU? It seems like MGLRU set active flag in
> > > folio_add_lru().
> > 
> > Yes. If you are referring to this set_active, I think it is
> > incorrect, so I have fixed it here and am waiting for review:
> > 
> > https://lore.kernel.org/linux-mm/20260418120233.7162-1-baohua@kernel.org/
> > 
> > > 
> > > JP, we need to clean active flag but let's check what else can be set before
> > > folio_add_lru().
> 
> Barry/Shakeel,
> 
> We can do something like this as a special case for MGLRU:
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index 71607b0ce3d18..68ea929f65031 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -185,6 +185,8 @@ static void folio_batch_move_lru(struct folio_batch
> *fbatch, move_fn_t move_fn)
>  		 * deferred split list to avoid a dangling list entry.
>  		 */
>  		if (is_lru_add && folio_ref_freeze(folio, 1)) {
> +			if (lru_gen_enabled())
> +				__folio_clear_active(folio);
>  			folio_unqueue_deferred_split(folio);
>  			fbatch->folios[i] = NULL;
>  			folio_batch_add(&free_fbatch, folio);
> 
> Unless Barry's patch works out... Any thoughts?

I think this is fine. We can remove this later when Barry's patch is settled in
a followup path.

>