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

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

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().

JP, we need to clean active flag but let's check what else can be set before
folio_add_lru().