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AJvYcCUI75dXXjif2sHReaGgSv9BLMrbu5OpQCiqlY8HqFSul67X1ZsDCwFPOcaiyEWVhKZHA9GxxxmD8dh4i/NIJR5Wutk= X-Gm-Message-State: AOJu0Yxhp5FRHmuKimxP8Q9ePeDtaIThO+m9RqGmJvQwMsJ4sw5ztB0T GmnkznEdqJodUfDHGO5dn4kOiQLHRGkzzWsndlwLK98Hi6xrwpEJ5w7a+sbwIXQXkmjxW4bZhu9 0l3MjpKih2u64m+OXa19VkTi2kyTR316Qr14= X-Google-Smtp-Source: AGHT+IEhUTM6oI3+Rfq1ybG03zOsE2x5klLXRrIDyw8RVBJS1ZZi3l/2IZtGqmwAMh80+f/dxB4MtSbpfbZYlcdcyrw= X-Received: by 2002:a05:6102:3708:b0:47a:41b5:ee72 with SMTP id s8-20020a056102370800b0047a41b5ee72mr14450495vst.20.1713317728435; Tue, 16 Apr 2024 18:35:28 -0700 (PDT) MIME-Version: 1.0 References: <20240409082631.187483-1-21cnbao@gmail.com> <20240409082631.187483-2-21cnbao@gmail.com> <87y19f2lq3.fsf@yhuang6-desk2.ccr.corp.intel.com> <87jzkz2g3t.fsf@yhuang6-desk2.ccr.corp.intel.com> <87bk6b2elo.fsf@yhuang6-desk2.ccr.corp.intel.com> <877cgy2ifu.fsf@yhuang6-desk2.ccr.corp.intel.com> <87ttk20zns.fsf@yhuang6-desk2.ccr.corp.intel.com> <87jzkw25hl.fsf@yhuang6-desk2.ccr.corp.intel.com> In-Reply-To: <87jzkw25hl.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Barry Song <21cnbao@gmail.com> Date: Wed, 17 Apr 2024 13:35:17 +1200 Message-ID: Subject: Re: [PATCH v2 1/5] mm: swap: introduce swap_free_nr() for batched swap_free() To: "Huang, Ying" Cc: akpm@linux-foundation.org, linux-mm@kvack.org, baolin.wang@linux.alibaba.com, chrisl@kernel.org, david@redhat.com, hanchuanhua@oppo.com, hannes@cmpxchg.org, hughd@google.com, kasong@tencent.com, ryan.roberts@arm.com, surenb@google.com, v-songbaohua@oppo.com, willy@infradead.org, xiang@kernel.org, yosryahmed@google.com, yuzhao@google.com, ziy@nvidia.com, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 7B8D0180009 X-Stat-Signature: iq4j8dcwyha7af35n1dx9fiuz68fj76a X-HE-Tag: 1713317729-134992 X-HE-Meta: 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 VNeRsBKX 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Apr 17, 2024 at 12:34=E2=80=AFPM Huang, Ying = wrote: > > Barry Song <21cnbao@gmail.com> writes: > > > On Tue, Apr 16, 2024 at 3:13=E2=80=AFPM Huang, Ying wrote: > >> > >> Barry Song <21cnbao@gmail.com> writes: > >> > >> > On Tue, Apr 16, 2024 at 1:42=E2=80=AFPM Huang, Ying wrote: > >> >> > >> >> Barry Song <21cnbao@gmail.com> writes: > >> >> > >> >> > On Mon, Apr 15, 2024 at 8:53=E2=80=AFPM Huang, Ying wrote: > >> >> >> > >> >> >> Barry Song <21cnbao@gmail.com> writes: > >> >> >> > >> >> >> > On Mon, Apr 15, 2024 at 8:21=E2=80=AFPM Huang, Ying wrote: > >> >> >> >> > >> >> >> >> Barry Song <21cnbao@gmail.com> writes: > >> >> >> >> > >> >> >> >> > On Mon, Apr 15, 2024 at 6:19=E2=80=AFPM Huang, Ying wrote: > >> >> >> >> >> > >> >> >> >> >> Barry Song <21cnbao@gmail.com> writes: > >> >> >> >> >> > >> >> >> >> >> > From: Chuanhua Han > >> >> >> >> >> > > >> >> >> >> >> > While swapping in a large folio, we need to free swaps r= elated to the whole > >> >> >> >> >> > folio. To avoid frequently acquiring and releasing swap = locks, it is better > >> >> >> >> >> > to introduce an API for batched free. > >> >> >> >> >> > > >> >> >> >> >> > Signed-off-by: Chuanhua Han > >> >> >> >> >> > Co-developed-by: Barry Song > >> >> >> >> >> > Signed-off-by: Barry Song > >> >> >> >> >> > --- > >> >> >> >> >> > include/linux/swap.h | 5 +++++ > >> >> >> >> >> > mm/swapfile.c | 51 +++++++++++++++++++++++++++++= +++++++++++++++ > >> >> >> >> >> > 2 files changed, 56 insertions(+) > >> >> >> >> >> > > >> >> >> >> >> > diff --git a/include/linux/swap.h b/include/linux/swap.h > >> >> >> >> >> > index 11c53692f65f..b7a107e983b8 100644 > >> >> >> >> >> > --- a/include/linux/swap.h > >> >> >> >> >> > +++ b/include/linux/swap.h > >> >> >> >> >> > @@ -483,6 +483,7 @@ extern void swap_shmem_alloc(swp_ent= ry_t); > >> >> >> >> >> > extern int swap_duplicate(swp_entry_t); > >> >> >> >> >> > extern int swapcache_prepare(swp_entry_t); > >> >> >> >> >> > extern void swap_free(swp_entry_t); > >> >> >> >> >> > +extern void swap_free_nr(swp_entry_t entry, int nr_page= s); > >> >> >> >> >> > extern void swapcache_free_entries(swp_entry_t *entries= , int n); > >> >> >> >> >> > extern void free_swap_and_cache_nr(swp_entry_t entry, i= nt nr); > >> >> >> >> >> > int swap_type_of(dev_t device, sector_t offset); > >> >> >> >> >> > @@ -564,6 +565,10 @@ static inline void swap_free(swp_en= try_t swp) > >> >> >> >> >> > { > >> >> >> >> >> > } > >> >> >> >> >> > > >> >> >> >> >> > +void swap_free_nr(swp_entry_t entry, int nr_pages) > >> >> >> >> >> > +{ > >> >> >> >> >> > +} > >> >> >> >> >> > + > >> >> >> >> >> > static inline void put_swap_folio(struct folio *folio, = swp_entry_t swp) > >> >> >> >> >> > { > >> >> >> >> >> > } > >> >> >> >> >> > diff --git a/mm/swapfile.c b/mm/swapfile.c > >> >> >> >> >> > index 28642c188c93..f4c65aeb088d 100644 > >> >> >> >> >> > --- a/mm/swapfile.c > >> >> >> >> >> > +++ b/mm/swapfile.c > >> >> >> >> >> > @@ -1356,6 +1356,57 @@ void swap_free(swp_entry_t entry) > >> >> >> >> >> > __swap_entry_free(p, entry); > >> >> >> >> >> > } > >> >> >> >> >> > > >> >> >> >> >> > +/* > >> >> >> >> >> > + * Free up the maximum number of swap entries at once t= o limit the > >> >> >> >> >> > + * maximum kernel stack usage. > >> >> >> >> >> > + */ > >> >> >> >> >> > +#define SWAP_BATCH_NR (SWAPFILE_CLUSTER > 512 ? 512 : S= WAPFILE_CLUSTER) > >> >> >> >> >> > + > >> >> >> >> >> > +/* > >> >> >> >> >> > + * Called after swapping in a large folio, > >> >> >> >> >> > >> >> >> >> >> IMHO, it's not good to document the caller in the function= definition. > >> >> >> >> >> Because this will discourage function reusing. > >> >> >> >> > > >> >> >> >> > ok. right now there is only one user that is why it is adde= d. but i agree > >> >> >> >> > we can actually remove this. > >> >> >> >> > > >> >> >> >> >> > >> >> >> >> >> > batched free swap entries > >> >> >> >> >> > + * for this large folio, entry should be for the first = subpage and > >> >> >> >> >> > + * its offset is aligned with nr_pages > >> >> >> >> >> > >> >> >> >> >> Why do we need this? > >> >> >> >> > > >> >> >> >> > This is a fundamental requirement for the existing kernel, = folio's > >> >> >> >> > swap offset is naturally aligned from the first moment add_= to_swap > >> >> >> >> > to add swapcache's xa. so this comment is describing the ex= isting > >> >> >> >> > fact. In the future, if we want to support swap-out folio t= o discontiguous > >> >> >> >> > and not-aligned offsets, we can't pass entry as the paramet= er, we should > >> >> >> >> > instead pass ptep or another different data struct which ca= n connect > >> >> >> >> > multiple discontiguous swap offsets. > >> >> >> >> > > >> >> >> >> > I feel like we only need "for this large folio, entry shoul= d be for > >> >> >> >> > the first subpage" and drop "and its offset is aligned with= nr_pages", > >> >> >> >> > the latter is not important to this context at all. > >> >> >> >> > >> >> >> >> IIUC, all these are requirements of the only caller now, not = the > >> >> >> >> function itself. If only part of the all swap entries of a m= THP are > >> >> >> >> called with swap_free_nr(), can swap_free_nr() still do its w= ork? If > >> >> >> >> so, why not make swap_free_nr() as general as possible? > >> >> >> > > >> >> >> > right , i believe we can make swap_free_nr() as general as pos= sible. > >> >> >> > > >> >> >> >> > >> >> >> >> >> > >> >> >> >> >> > + */ > >> >> >> >> >> > +void swap_free_nr(swp_entry_t entry, int nr_pages) > >> >> >> >> >> > +{ > >> >> >> >> >> > + int i, j; > >> >> >> >> >> > + struct swap_cluster_info *ci; > >> >> >> >> >> > + struct swap_info_struct *p; > >> >> >> >> >> > + unsigned int type =3D swp_type(entry); > >> >> >> >> >> > + unsigned long offset =3D swp_offset(entry); > >> >> >> >> >> > + int batch_nr, remain_nr; > >> >> >> >> >> > + DECLARE_BITMAP(usage, SWAP_BATCH_NR) =3D { 0 }; > >> >> >> >> >> > + > >> >> >> >> >> > + /* all swap entries are within a cluster for mTHP = */ > >> >> >> >> >> > + VM_BUG_ON(offset % SWAPFILE_CLUSTER + nr_pages > S= WAPFILE_CLUSTER); > >> >> >> >> >> > + > >> >> >> >> >> > + if (nr_pages =3D=3D 1) { > >> >> >> >> >> > + swap_free(entry); > >> >> >> >> >> > + return; > >> >> >> >> >> > + } > >> >> >> >> >> > >> >> >> >> >> Is it possible to unify swap_free() and swap_free_nr() int= o one function > >> >> >> >> >> with acceptable performance? IIUC, the general rule in mT= HP effort is > >> >> >> >> >> to avoid duplicate functions between mTHP and normal small= folio. > >> >> >> >> >> Right? > >> >> >> >> > > >> >> >> >> > I don't see why. > >> >> >> >> > >> >> >> >> Because duplicated implementation are hard to maintain in the= long term. > >> >> >> > > >> >> >> > sorry, i actually meant "I don't see why not", for some reaso= n, the "not" > >> >> >> > was missed. Obviously I meant "why not", there was a "but" aft= er it :-) > >> >> >> > > >> >> >> >> > >> >> >> >> > but we have lots of places calling swap_free(), we may > >> >> >> >> > have to change them all to call swap_free_nr(entry, 1); the= other possible > >> >> >> >> > way is making swap_free() a wrapper of swap_free_nr() alway= s using > >> >> >> >> > 1 as the argument. In both cases, we are changing the seman= tics of > >> >> >> >> > swap_free_nr() to partially freeing large folio cases and h= ave to drop > >> >> >> >> > "entry should be for the first subpage" then. > >> >> >> >> > > >> >> >> >> > Right now, the semantics is > >> >> >> >> > * swap_free_nr() for an entire large folio; > >> >> >> >> > * swap_free() for one entry of either a large folio or a sm= all folio > >> >> >> >> > >> >> >> >> As above, I don't think the these semantics are important for > >> >> >> >> swap_free_nr() implementation. > >> >> >> > > >> >> >> > right. I agree. If we are ready to change all those callers, n= othing > >> >> >> > can stop us from removing swap_free(). > >> >> >> > > >> >> >> >> > >> >> >> >> >> > >> >> >> >> >> > + > >> >> >> >> >> > + remain_nr =3D nr_pages; > >> >> >> >> >> > + p =3D _swap_info_get(entry); > >> >> >> >> >> > + if (p) { > >> >> >> >> >> > + for (i =3D 0; i < nr_pages; i +=3D batch_n= r) { > >> >> >> >> >> > + batch_nr =3D min_t(int, SWAP_BATCH= _NR, remain_nr); > >> >> >> >> >> > + > >> >> >> >> >> > + ci =3D lock_cluster_or_swap_info(p= , offset); > >> >> >> >> >> > + for (j =3D 0; j < batch_nr; j++) { > >> >> >> >> >> > + if (__swap_entry_free_lock= ed(p, offset + i * SWAP_BATCH_NR + j, 1)) > >> >> >> >> >> > + __bitmap_set(usage= , j, 1); > >> >> >> >> >> > + } > >> >> >> >> >> > + unlock_cluster_or_swap_info(p, ci)= ; > >> >> >> >> >> > + > >> >> >> >> >> > + for_each_clear_bit(j, usage, batch= _nr) > >> >> >> >> >> > + free_swap_slot(swp_entry(t= ype, offset + i * SWAP_BATCH_NR + j)); > >> >> >> >> >> > + > >> >> >> >> >> > + bitmap_clear(usage, 0, SWAP_BATCH_= NR); > >> >> >> >> >> > + remain_nr -=3D batch_nr; > >> >> >> >> >> > + } > >> >> >> >> >> > + } > >> >> >> >> >> > +} > >> >> >> >> >> > + > >> >> >> >> >> > /* > >> >> >> >> >> > * Called after dropping swapcache to decrease refcnt t= o swap entries. > >> >> >> >> >> > */ > >> >> >> >> >> > >> >> >> >> >> put_swap_folio() implements batching in another method. D= o you think > >> >> >> >> >> that it's good to use the batching method in that function= here? It > >> >> >> >> >> avoids to use bitmap operations and stack space. > >> >> >> >> > > >> >> >> >> > Chuanhua has strictly limited the maximum stack usage to se= veral > >> >> >> >> > unsigned long, > >> >> >> >> > >> >> >> >> 512 / 8 =3D 64 bytes. > >> >> >> >> > >> >> >> >> So, not trivial. > >> >> >> >> > >> >> >> >> > so this should be safe. on the other hand, i believe this > >> >> >> >> > implementation is more efficient, as put_swap_folio() migh= t lock/ > >> >> >> >> > unlock much more often whenever __swap_entry_free_locked re= turns > >> >> >> >> > 0. > >> >> >> >> > >> >> >> >> There are 2 most common use cases, > >> >> >> >> > >> >> >> >> - all swap entries have usage count =3D=3D 0 > >> >> >> >> - all swap entries have usage count !=3D 0 > >> >> >> >> > >> >> >> >> In both cases, we only need to lock/unlock once. In fact, I = didn't > >> >> >> >> find possible use cases other than above. > >> >> >> > > >> >> >> > i guess the point is free_swap_slot() shouldn't be called with= in > >> >> >> > lock_cluster_or_swap_info? so when we are freeing nr_pages slo= ts, > >> >> >> > we'll have to unlock and lock nr_pages times? and this is the= most > >> >> >> > common scenario. > >> >> >> > >> >> >> No. In put_swap_folio(), free_entries is either SWAPFILE_CLUSTE= R (that > >> >> >> is, nr_pages) or 0. These are the most common cases. > >> >> >> > >> >> > > >> >> > i am actually talking about the below code path, > >> >> > > >> >> > void put_swap_folio(struct folio *folio, swp_entry_t entry) > >> >> > { > >> >> > ... > >> >> > > >> >> > ci =3D lock_cluster_or_swap_info(si, offset); > >> >> > ... > >> >> > for (i =3D 0; i < size; i++, entry.val++) { > >> >> > if (!__swap_entry_free_locked(si, offset + i, SWA= P_HAS_CACHE)) { > >> >> > unlock_cluster_or_swap_info(si, ci); > >> >> > free_swap_slot(entry); > >> >> > if (i =3D=3D size - 1) > >> >> > return; > >> >> > lock_cluster_or_swap_info(si, offset); > >> >> > } > >> >> > } > >> >> > unlock_cluster_or_swap_info(si, ci); > >> >> > } > >> >> > > >> >> > but i guess you are talking about the below code path: > >> >> > > >> >> > void put_swap_folio(struct folio *folio, swp_entry_t entry) > >> >> > { > >> >> > ... > >> >> > > >> >> > ci =3D lock_cluster_or_swap_info(si, offset); > >> >> > if (size =3D=3D SWAPFILE_CLUSTER) { > >> >> > map =3D si->swap_map + offset; > >> >> > for (i =3D 0; i < SWAPFILE_CLUSTER; i++) { > >> >> > val =3D map[i]; > >> >> > VM_BUG_ON(!(val & SWAP_HAS_CACHE)); > >> >> > if (val =3D=3D SWAP_HAS_CACHE) > >> >> > free_entries++; > >> >> > } > >> >> > if (free_entries =3D=3D SWAPFILE_CLUSTER) { > >> >> > unlock_cluster_or_swap_info(si, ci); > >> >> > spin_lock(&si->lock); > >> >> > mem_cgroup_uncharge_swap(entry, SWAPFILE_= CLUSTER); > >> >> > swap_free_cluster(si, idx); > >> >> > spin_unlock(&si->lock); > >> >> > return; > >> >> > } > >> >> > } > >> >> > } > >> >> > >> >> I am talking about both code paths. In 2 most common cases, > >> >> __swap_entry_free_locked() will return 0 or !0 for all entries in r= ange. > >> > > >> > I grasp your point, but if conditions involving 0 or non-0 values fa= il, we'll > >> > end up repeatedly unlocking and locking. Picture a scenario with a l= arge > >> > folio shared by multiple processes. One process might unmap a portio= n > >> > while another still holds an entire mapping to it. This could lead t= o situations > >> > where free_entries doesn't equal 0 and free_entries doesn't equal > >> > nr_pages, resulting in multiple unlock and lock operations. > >> > >> This is impossible in current caller, because the folio is in the swap > >> cache. But if we move the change to __swap_entry_free_nr(), we may ru= n > >> into this situation. > > > > I don't understand why it is impossible, after try_to_unmap_one() has d= one > > on one process, mprotect and munmap called on a part of the large folio > > pte entries which now have been swap entries, we are removing the PTE > > for this part. Another process can entirely hit the swapcache and have > > all swap entries mapped there, and we call swap_free_nr(entry, nr_pages= ) in > > do_swap_page. Within those swap entries, some have swapcount=3D1 and ot= hers > > have swapcount > 1. Am I missing something? > > For swap entries with swapcount=3D1, its sis->swap_map[] will be > > 1 | SWAP_HAS_CACHE > > so, __swap_entry_free_locked() will return SWAP_HAS_CACHE instead of 0. > > The swap entries will be free in > > folio_free_swap > delete_from_swap_cache > put_swap_folio > Yes. I realized this after replying to you yesterday. > >> > Chuanhua has invested significant effort in following Ryan's suggest= ion > >> > for the current approach, which generally handles all cases, especia= lly > >> > partial unmapping. Additionally, the widespread use of swap_free_nr(= ) > >> > as you suggested across various scenarios is noteworthy. > >> > > >> > Unless there's evidence indicating performance issues or bugs, I bel= ieve > >> > the current approach remains preferable. > >> > >> TBH, I don't like the large stack space usage (64 bytes). How about u= se > >> a "unsigned long" as bitmap? Then, we use much less stack space, use > >> bitmap =3D=3D 0 and bitmap =3D=3D (unsigned long)(-1) to check the mos= t common > >> use cases. And, we have enough batching. > > > > that is quite a straightforward modification like, > > > > - #define SWAP_BATCH_NR (SWAPFILE_CLUSTER > 512 ? 512 : SWAPFILE_CLUSTE= R) > > + #define SWAP_BATCH_NR (SWAPFILE_CLUSTER > 64 ? 64 : SWAPFILE_CLUSTER) > > > > there is no necessity to remove the bitmap API and move to raw > > unsigned long operations. > > as bitmap is exactly some unsigned long. on 64bit CPU, we are now one > > unsigned long, > > on 32bit CPU, it is now two unsigned long. > > Yes. We can still use most bitmap APIs if we use "unsigned long" as > bitmap. The advantage of "unsigned long" is to guarantee that > bitmap_empty() and bitmap_full() is trivial. We can use that for > optimization. For example, we can skip unlock/lock if bitmap_empty(). anyway we have avoided lock_cluster_or_swap_info and unlock_cluster_or_swap= _info for each individual swap entry. if bitma_empty(), we won't call free_swap_slot() so no chance to further take any lock, right? the optimization of bitmap_full() seems to be more useful only after we hav= e void free_swap_slot(swp_entry_t entry, int nr) in which we can avoid many spin_lock_irq(&cache->free_lock); On the other hand, it seems we can directly call swapcache_free_entries() to skip cache if nr_pages >=3D SWAP_BATCH(64) this might be an optimization as we are now having a bitmap exactly equals 64. > > >> > >> >> > >> >> > we are mTHP, so we can't assume our size is SWAPFILE_CLUSTER? > >> >> > or you want to check free_entries =3D=3D "1 << swap_entry_order(f= olio_order(folio))" > >> >> > instead of SWAPFILE_CLUSTER for the "for (i =3D 0; i < size; i++,= entry.val++)" > >> >> > path? > >> >> > >> >> Just replace SWAPFILE_CLUSTER with "nr_pages" in your code. > >> >> > >> >> > > >> >> >> >> > >> >> >> >> And, we should add batching in __swap_entry_free(). That wil= l help > >> >> >> >> free_swap_and_cache_nr() too. > >> >> > > >> >> > Chris Li and I actually discussed it before, while I completely > >> >> > agree this can be batched. but i'd like to defer this as an incre= mental > >> >> > patchset later to keep this swapcache-refault small. > >> >> > >> >> OK. > >> >> > >> >> >> > >> >> >> Please consider this too. > > -- > Best Regards, > Huang, Ying