> -----Original Message----- > From: Chengming Zhou <chengming.zhou@xxxxxxxxx> > Sent: Monday, December 2, 2024 7:06 PM > To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>; Yosry Ahmed > <yosryahmed@xxxxxxxxxx> > Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; > hannes@xxxxxxxxxxx; nphamcs@xxxxxxxxx; usamaarif642@xxxxxxxxx; > ryan.roberts@xxxxxxx; 21cnbao@xxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; > Feghali, Wajdi K <wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh > <vinodh.gopal@xxxxxxxxx> > Subject: Re: [PATCH v1 2/2] mm: zswap: zswap_store_pages() simplifications > for batching. > > On 2024/12/3 09:01, Sridhar, Kanchana P wrote: > > Hi Chengming, Yosry, > > > >> -----Original Message----- > >> From: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > >> Sent: Monday, December 2, 2024 11:33 AM > >> To: Chengming Zhou <chengming.zhou@xxxxxxxxx> > >> Cc: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>; linux- > >> kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; hannes@xxxxxxxxxxx; > >> nphamcs@xxxxxxxxx; usamaarif642@xxxxxxxxx; ryan.roberts@xxxxxxx; > >> 21cnbao@xxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; Feghali, Wajdi K > >> <wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh <vinodh.gopal@xxxxxxxxx> > >> Subject: Re: [PATCH v1 2/2] mm: zswap: zswap_store_pages() > simplifications > >> for batching. > >> > >> On Wed, Nov 27, 2024 at 11:00 PM Chengming Zhou > >> <chengming.zhou@xxxxxxxxx> wrote: > >>> > >>> On 2024/11/28 06:53, Kanchana P Sridhar wrote: > >>>> In order to set up zswap_store_pages() to enable a clean batching > >>>> implementation in [1], this patch implements the following changes: > >>>> > >>>> 1) Addition of zswap_alloc_entries() which will allocate zswap entries for > >>>> all pages in the specified range for the folio, upfront. If this fails, > >>>> we return an error status to zswap_store(). > >>>> > >>>> 2) Addition of zswap_compress_pages() that calls zswap_compress() for > >> each > >>>> page, and returns false if any zswap_compress() fails, so > >>>> zswap_store_page() can cleanup resources allocated and return an > >> error > >>>> status to zswap_store(). > >>>> > >>>> 3) A "store_pages_failed" label that is a catch-all for all failure points > >>>> in zswap_store_pages(). This facilitates cleaner error handling within > >>>> zswap_store_pages(), which will become important for IAA compress > >>>> batching in [1]. > >>>> > >>>> [1]: https://patchwork.kernel.org/project/linux- > mm/list/?series=911935 > >>>> > >>>> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@xxxxxxxxx> > >>>> --- > >>>> mm/zswap.c | 93 +++++++++++++++++++++++++++++++++++++++++- > --- > >> --------- > >>>> 1 file changed, 71 insertions(+), 22 deletions(-) > >>>> > >>>> diff --git a/mm/zswap.c b/mm/zswap.c > >>>> index b09d1023e775..db80c66e2205 100644 > >>>> --- a/mm/zswap.c > >>>> +++ b/mm/zswap.c > >>>> @@ -1409,9 +1409,56 @@ static void shrink_worker(struct work_struct > >> *w) > >>>> * main API > >>>> **********************************/ > >>>> > >>>> +static bool zswap_compress_pages(struct page *pages[], > >>>> + struct zswap_entry *entries[], > >>>> + u8 nr_pages, > >>>> + struct zswap_pool *pool) > >>>> +{ > >>>> + u8 i; > >>>> + > >>>> + for (i = 0; i < nr_pages; ++i) { > >>>> + if (!zswap_compress(pages[i], entries[i], pool)) > >>>> + return false; > >>>> + } > >>>> + > >>>> + return true; > >>>> +} > >>> > >>> How about introducing a `zswap_compress_folio()` interface which > >>> can be used by `zswap_store()`? > >>> ``` > >>> zswap_store() > >>> nr_pages = folio_nr_pages(folio) > >>> > >>> entries = zswap_alloc_entries(nr_pages) > >>> > >>> ret = zswap_compress_folio(folio, entries, pool) > >>> > >>> // store entries into xarray and LRU list > >>> ``` > >>> > >>> And this version `zswap_compress_folio()` is very simple for now: > >>> ``` > >>> zswap_compress_folio() > >>> nr_pages = folio_nr_pages(folio) > >>> > >>> for (index = 0; index < nr_pages; ++index) { > >>> struct page *page = folio_page(folio, index); > >>> > >>> if (!zswap_compress(page, &entries[index], pool)) > >>> return false; > >>> } > >>> > >>> return true; > >>> ``` > >>> This can be easily extended to support your "batched" version. > >>> > >>> Then the old `zswap_store_page()` could be removed. > >>> > >>> The good point is simplicity, that we don't need to slice folio > >>> into multiple batches, then repeat the common operations for each > >>> batch, like preparing entries, storing into xarray and LRU list... > >> > >> +1 > > > > Thanks for the code review comments. One question though: would > > it make sense to trade-off the memory footprint cost with the code > > simplification? For instance, lets say we want to store a 64k folio. > > We would allocate memory for 16 zswap entries, and lets say one of > > the compressions fails, we would deallocate memory for all 16 zswap > > entries. Could this lead to zswap_entry kmem_cache starvation and > > subsequent zswap_store() failures in multiple processes scenarios? > > Ah, I get your consideration. But it's the unlikely case, right? > > If the case you mentioned above happens a lot, I think yes, we should > optimize its memory footprint to avoid allocation and deallocation. Thanks Chengming. I see your point. Let me gather performance data for the two options, and share. > > On the other hand, we should consider a folio would be compressed > successfully in most cases. So we have to allocate all entries > eventually. > > Based on your consideration, I think your way is ok too, although > I think the patch 2/2 should be dropped, since it hides pages loop > in smaller functions, as Yosry mentioned too. My main intent with patch 2/2 was to set up the error handling path to be common and simpler, whether errors were encountered during compression/zpool_malloc/xarray store. Hence, I initialize the allocated zswap_entry's handle in zswap_alloc_entries() to ERR_PTR(-EINVAL), so it is easy for the common error handling code in patch 2 to determine if the handle was allocated (and hence needs to be freed). This benefits the batching code by eliminating the need to maintain state as to which stage of zswap_store_pages() sees an error, based on which resources would need to be deleted. My key consideration is to keep the batching error handling code simple, hence these changes in patch 2. The changes described above would help batching, and should not impact the non-batching case, as indicated by the regression testing data I've included in the cover letter. I don't mind inlining the implementation of the helper functions, as I mentioned in my response to Yosry. I am hoping the error handling simplifications are acceptable, since they will help the batching code. > > > > > In other words, allocating entries in smaller batches -- more specifically, > > only the compress batchsize -- seems to strike a balance in terms of > > memory footprint, while mitigating the starvation aspect, and possibly > > also helping latency (allocating a large # of zswap entries and potentially > > deallocating, could impact latency). > > If we consider the likely case (compress successfully), the whole > latency should be better, right? Since we can bulk allocate all > entries at first, and bulk insert to xarray and LRU at last. I think so too, but would like to confirm with some experiments and update. > > > > > If we agree with the merits of processing a large folio in smaller batches: > > this in turn requires we store the smaller batches of entries in the > > xarray/LRU before moving to the next batch. Which means all the > > zswap_store() ops need to be done for a batch before moving to the next > > batch. > > > > Both way is ok for me based on your memory footprint consideration > above. Sounds good, thanks! Thanks, Kanchana > > Thanks.
