> On Dec 17, 2021, at 9:03 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Sat, Dec 18, 2021 at 04:52:13AM +0000, Nadav Amit wrote:
>> Take for instance memcached and assume you overcommit memory with a very fast
>> swap (e.g., pmem, zram, perhaps even slower). Now, it turns out memcached
>> often accesses a page first for read and shortly after for write. I
>> encountered, in a similar scenario, that the page reference that
>> lru_cache_add() takes during the first faultin event (for read), causes a COW
>> on a write page-fault that happens shortly after [1]. So on memcached I
>> assume this would also trigger frequent unnecessary COWs.
>
> Why are we comparing page_count() against 1 and not 1 + PageLRU(page)?
> Having a reference from the LRU should be expected. Is it because of
> some race that we'd need to take the page lock to protect against?
>
IIUC, the reference that is taken on the page is taken before SetPageLRU()
is called and the reference is later dropped:
lru_add_drain()
lru_add_drain_cpu()
__pagevec_lru_add()
__pagevec_lru_add_fn()
__pagevec_lru_add_fn()
SetPageLRU() <- sets the LRU
release_pages() <- drops the reference
It is one scenario I encountered. There might be others that take transient
references on pages that cause unnecessary COWs. I think David and Andrea
had few in mind. To trigger a COW bug I once used mlock()/munlock() that
take such transient reference. But who knows how many other cases exist
(KSM? vmscan?)
