> -----Original Message----- > From: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > Sent: Thursday, September 26, 2024 10:20 AM > To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; > linux-mm@xxxxxxxxx; nphamcs@xxxxxxxxx; chengming.zhou@xxxxxxxxx; > usamaarif642@xxxxxxxxx; shakeel.butt@xxxxxxxxx; ryan.roberts@xxxxxxx; > Huang, Ying <ying.huang@xxxxxxxxx>; 21cnbao@xxxxxxxxx; akpm@linux- > foundation.org; Zou, Nanhai <nanhai.zou@xxxxxxxxx>; Feghali, Wajdi K > <wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh <vinodh.gopal@xxxxxxxxx> > Subject: Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in > zswap_store(). > > On Thu, Sep 26, 2024 at 9:40 AM Sridhar, Kanchana P > <kanchana.p.sridhar@xxxxxxxxx> wrote: > > > > > -----Original Message----- > > > From: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > > > Sent: Wednesday, September 25, 2024 9:52 PM > > > To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx> > > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>; linux- > kernel@xxxxxxxxxxxxxxx; > > > linux-mm@xxxxxxxxx; nphamcs@xxxxxxxxx; chengming.zhou@xxxxxxxxx; > > > usamaarif642@xxxxxxxxx; shakeel.butt@xxxxxxxxx; > ryan.roberts@xxxxxxx; > > > Huang, Ying <ying.huang@xxxxxxxxx>; 21cnbao@xxxxxxxxx; akpm@linux- > > > foundation.org; Zou, Nanhai <nanhai.zou@xxxxxxxxx>; Feghali, Wajdi K > > > <wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh <vinodh.gopal@xxxxxxxxx> > > > Subject: Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in > > > zswap_store(). > > > > > > [..] > > > > > > > > One thing I realized while reworking the patches for the batched checks > is: > > > > within zswap_store_page(), we set the entry->objcg and entry->pool > before > > > > adding it to the xarray. Given this, wouldn't it be safer to get the objcg > > > > and pool reference per sub-page, locally in zswap_store_page(), rather > than > > > > obtaining batched references at the end if the store is successful? If we > > > want > > > > zswap_store_page() to be self-contained and correct as far as the entry > > > > being created and added to the xarray, it seems like the right thing to > do? > > > > I am a bit apprehensive about the entry being added to the xarray > without > > > > a reference obtained on the objcg and pool, because any page- > > > faults/writeback > > > > that occur on sub-pages added to the xarray before the entire folio has > been > > > > stored, would run into issues. > > > > > > We definitely should not obtain references to the pool and objcg after > > > initializing the entries with them. We can obtain all references in > > > zswap_store() before zswap_store_page(). IOW, the batching in this > > > case should be done before the per-page operations, not after. > > > > Thanks Yosry. IIUC, we should obtain all references to the objcg and to the > > zswap_pool at the start of zswap_store. > > > > In the case of error on any sub-page, we will unwind state for potentially > > only the stored pages or the entire folio if it happened to already be in > zswap > > and is being re-written. We might need some additional book-keeping to > > keep track of which sub-pages were found in the xarray and > zswap_entry_free() > > got called (nr_sb). Assuming I define a new "obj_cgroup_put_many()", I > would need > > to call this with (folio_nr_pages() - nr_sb). > > > > As far as zswap_pool_get(), there is some added complexity if we want to > > keep the existing implementation that calls "percpu_ref_tryget()", and > assuming > > this is extended to provide a new "zswap_pool_get_many()" that calls > > "percpu_ref_tryget_many()". Is there a reason we use percpu_ref_tryget() > instead > > of percpu_ref_get()? Reason I ask is, with tryget(), if for some reason the > pool->ref > > is 0, no further increments will be made. If so, upon unwinding state in > > zswap_store(), I would need to special-case to catch this before calling a > new > > "zswap_pool_put_many()". > > > > Things could be a little simpler if zswap_pool_get() can use > "percpu_ref_get()" > > which will always increment the refcount. Since the zswap pool->ref is > initialized > > to "1", this seems Ok, but I don't know if there will be unintended > consequences. > > > > Can you please advise on what is the simplest/cleanest approach: > > > > 1) Proceed with the above changes without changing percpu_ref_tryget in > > zswap_pool_get. Needs special-casing in zswap_store to detect pool- > >ref > > being "0" before calling zswap_pool_put[_many]. > > My assumption is that we can reorder the code such that if > zswap_pool_get_many() fails we don't call zswap_pool_put_many() to > begin with (e.g. jump to a label after zswap_pool_put_many()). However, the pool refcount could change between the start and end of zswap_store. > > > 2) Modify zswap_pool_get/zswap_pool_get_many to use > percpu_ref_get_many > > and avoid special-casing to detect pool->ref being "0" before calling > > zswap_pool_put[_many]. > > I don't think we can simply switch the tryget to a get, as I believe > we can race with the pool being destroyed. That was my initial thought as well, but I figured this couldn't happen since the pool->ref is initialized to "1", and based on the existing implementation. In any case, I can understand the intent of the use of "tryget"; it is just that it adds to the considerations for reference batching. > > > 3) Keep the approach in v7 where obj_cgroup_get/put is localized to > > zswap_store_page for both success and error conditions, and any > unwinding > > state in zswap_store will take care of dropping references obtained from > > prior successful writes (from this or prior invocations of zswap_store). > > I am also fine with doing that and doing the reference batching as a follow up. I think so too! We could try and improve upon (3) with reference batching in a follow-up patch. Thanks, Kanchana > > > > > > Thanks, > > Kanchana > > > > > > > > > > > > > Just wanted to run this by you. The rest of the batched charging, atomic > > > > and stat updates should be Ok. > > > > > > > > Thanks, > > > > Kanchana > > > > > > > > > > > > > > Thanks, > > > > > Kanchana
