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dkim=pass header.d=google.com header.s=20210112 header.b=Uw0KXfvb; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf25.hostedemail.com: domain of yosryahmed@google.com designates 209.85.166.174 as permitted sender) smtp.mailfrom=yosryahmed@google.com X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 1BA44A0004 X-Stat-Signature: fg4fne3h99kthhnksj5y8mq1893cremb X-HE-Tag: 1667499084-528200 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: On Thu, Nov 3, 2022 at 11:08 AM Johannes Weiner wrote: > > On Thu, Nov 03, 2022 at 08:53:29AM -0700, Minchan Kim wrote: > > On Thu, Nov 03, 2022 at 11:18:04AM -0400, Johannes Weiner wrote: > > > On Wed, Nov 02, 2022 at 02:36:35PM -0700, Minchan Kim wrote: > > > > On Wed, Nov 02, 2022 at 12:28:56PM +0900, Sergey Senozhatsky wrote: > > > > > On (22/10/26 13:06), Nhat Pham wrote: > > > > > > struct size_class { > > > > > > - spinlock_t lock; > > > > > > struct list_head fullness_list[NR_ZS_FULLNESS]; > > > > > > /* > > > > > > * Size of objects stored in this class. Must be multiple > > > > > > @@ -247,8 +245,7 @@ struct zs_pool { > > > > > > #ifdef CONFIG_COMPACTION > > > > > > struct work_struct free_work; > > > > > > #endif > > > > > > - /* protect page/zspage migration */ > > > > > > - rwlock_t migrate_lock; > > > > > > + spinlock_t lock; > > > > > > }; > > > > > > > > > > I'm not in love with this, to be honest. One big pool lock instead > > > > > of 255 per-class locks doesn't look attractive, as one big pool lock > > > > > is going to be hammered quite a lot when zram is used, e.g. as a regular > > > > > block device with a file system and is under heavy parallel writes/reads. > > > > > > TBH the class always struck me as an odd scope to split the lock. Lock > > > contention depends on how variable the compression rate is of the > > > hottest incoming data, which is unpredictable from a user POV. > > > > > > My understanding is that the primary usecase for zram is swapping, and > > > the pool lock is the same granularity as the swap locking. > > > > People uses the zram to store caching object files in build server. > > Oh, interesting. We can try with a kernel build directory on zram. > > > > Do you have a particular one in mind? (I'm thinking journaled ones are > > > not of much interest, since their IO tends to be fairly serialized.) > > > > > > btrfs? > > > > I am not sure what FSes others are using but at least for me, just > > plain ext4. > > Okay, we can test with both. > > > > > I am also worry about that LRU stuff should be part of allocator > > > > instead of higher level. > > > > > > I'm sorry, but that's not a reasonable objection. > > > > > > These patches implement a core feature of being a zswap backend, using > > > standard LRU and locking techniques established by the other backends. > > > > > > I don't disagree that it would nicer if zswap had a strong abstraction > > > for backend pages and a generalized LRU. But that is major surgery on > > > a codebase of over 6,500 lines. It's not a reasonable ask to change > > > all that first before implementing a basic feature that's useful now. > > > > With same logic, folks added the LRU logic into their allocators > > without the effort considering moving the LRU into upper layer. > > > > And then trend is still going on since I have seen multiple times > > people are trying to add more allocators. So if it's not a reasonable > > ask to consier, we couldn't stop the trend in the end. > > So there is actually an ongoing effort to do that. Yosry and I have > spent quite some time on coming up with an LRU design that's > independent from compression policy over email and at Plumbers. > > My concern is more about the order of doing things: > > 1. The missing writeback support is a gaping hole in zsmalloc, which > affects production systems. A generalized LRU list is a good idea, > but it's a huge task that from a user pov really is not > critical. Even from a kernel dev / maintainer POV, there are bigger > fish to fry in the zswap code base and the backends than this. > > 2. Refactoring existing functionality is much easier than writing > generalized code that simultaneously enables new behavior. zsmalloc > is the most complex of our backends. To make its LRU writeback work > we had to patch zswap's ->map ordering to accomodate it, e.g. Such > tricky changes are easier to make and test incrementally. > > The generalized LRU project will hugely benefit from already having > a proven writeback implementation in zsmalloc, because then all the > requirements in zswap and zsmalloc will be in black and white. > > > > I get that your main interest is zram, and so this feature isn't of > > > interest to you. But zram isn't the only user, nor is it the primary > > > > I am interest to the feature but my interest is more of general swap > > layer to manage the LRU so that it could support any hierarchy among > > swap devices, not only zswap. > > I think we're on the same page about the longer term goals. > Yeah. As Johannes said, I was also recently looking into this. This can also help solve other problems than consolidating implementations. Currently if zswap rejects a page, it goes into swap, which is more-or-less a violation of page LRUs since hotter pages that are more recently reclaimed end up in swap (slow), while colder pages that were reclaimed before are in zswap. Having a separate layer managing the LRU of swap pages can also make sure this doesn't happen. More broadly, making zswap a separate layer from swap enables other improvements such as using zswap regardless of the presence of a backend swapfile and not consuming space in swapfiles if a page is in zswap. Of course, this is a much larger surgery. I am intending to spend more time looking further into this, but other things keep popping up :)