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 evLJ6/M3 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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 Mon, Apr 21, 2025 at 05:33:38PM +0100, Pedro Falcato wrote: > On Mon, Apr 21, 2025 at 10:47:39PM +0900, Harry Yoo wrote: > > Hi folks, > > > > Hi Harry, > > Some passing thoughts... Hi Pedro, thanks for taking a look. > > As a long term project, I'm starting to look into resurrecting > > Slab Movable Objects. The goal is to make certain types of slab memory > > movable and thus enable targeted reclamation, migration, and > > defragmentation. > > > > The main purpose of this posting is to briefly review what's been tried > > in the past, ask people why prior efforts have stalled (due to lack of > > time or insufficient justification for additional complexity?), > > and discuss what's feasible today. > > > > Please add anyone I may have missed to Cc. :) > > > > Previous Work on Slab Movable Objects > > ===================================== > > > > Christoph Lameter, Slab Defragmentation Reduction, 2007-2017 (V16: [2]): > > Christoph Lameter, Slab object migration for xarray, 2017-2018 (V2: [3]): > > Christoph's long-standing effort (since 2007) aiming to defragment > > slab memory in cases where sparsely populated slabs occupy excessive > > amount of memory. > > > > Early versions of the work focused on defragmenting slab caches > > for filesystem data structures such as inode, dentry, and buffer head. > > updatedb was suggested as the standard way to trigger for generating > > sparsely populated slabs on file servers. > > > > However, defragmenting slabs for filesystem data structures has proven > > to be very difficult to fully solve, because inodes and dentries are > > neither reclaimable nor migratable, limiting the effectiveness of > > defragmentation. > > > > In late 2018, the effort was revived with a new focus on migrating > > XArray nodes. However, it appears the work was discontinued after > > V2 [3]? > > > > Tobin C. Harding, Slab Movable Objects, 2019 (First Non-RFC: [5]) > > - Tobin C. Harding revived Christoph's earlier work and introduced > > a few enhancements, including partial shrinking of dentries, moving > > objects to and from a specific NUMA node, and balancing objects across > > all NUMA nodes. > > > > Also appears to be discontinued after the first non-RFC version [5]? > > > > At LSFMM 2017, Andrea Arcangeli suggested [6] virtually mapped slabs, > > which might be useful since migrating them does not require changing the > > address of objects. But as Rik van Riel pointed out at that time, it > > isn't really useful for defragmentation. Andrea Arcangeli responded > > that it can be beneficial for memory hotplug, compaction and out-of-memory > > avoidance. > > > > The exact mechanism wasn't described in [6], but I assume it'll involve > > 1) unmap a slab (and page faults after unmap need to wait for migration > > to complete), 2) copy objects to a new slab, and 3) map the new slab? > > But the idea hasn't gained enough attention for anyone to actually > > implement it. > > I don't think this is a silver bullet. It opens a whole separate can of worms > while maintaining similar issues. But instead of worrying about updating pointers, > you're worrying about locking out _any_ sort of access, which would involve stop_machine(). Haha, yes. When I read the LWN article I was like "Wait, can the kernel really synchronize access to slab objects while migrating the underlying pages?" and I sketched a very rough 'mecanism' in previous email without carefully considering correctness or feasibility. stop_machine() just to migrate slab objects sounds like a disaster. > You can't even atomically replace a PTE without running into issues (arm requires BBM, thus > this doesn't work) so it cannot be applied to anything that can't page fault (xarray and > the maple tree are used in IRQ paths, if I'm not mistaken). Yeah, while the idea is very simple, I can't think of any sane way to correctly implement this given that slab objects can be accessed in _any_ context. > > > > Potential Candidates of SMO > > =========================== > > > > Basic Rules > > ----------- > > > > - Slab memory can only be reclaimed or migrated if the user of the slab > > provides a way to isolate / migrate objects. > > - If objects can be reclaimed, it makes sense to simply reclaim them > > instead of migrating them (unless we know it's better to keep that > > object in memory). > > In any case I think you want to give subsystems the power to decide between > {RECLAIMED, MIGRATED, SKIPPED}. Totally agreed. > > - Some objects can't be reclaimed, but migrating them is (if possible) > > still useful for defragmentation and compaction. > > - However it is not always feasible > > > > Potential candidates include (but not limited to): > > -------------------------------------------------- > > > > - XArray nodes can be migrated (can't be reclaimed as they're being used) > > - Can be reclaimed if it only includes shadow entries. > > - Maple tree nodes (if without external locking) and VMAs can be migrated > > and obviously can't be reclaimed. > > - Negative dentry should be reclaimed, instead of being migrated. > > - Only unused dentries can be reclaimed without high cost. > > Unused dentries can also have a high cost if they're accessed in the future > (and the dentry LRU has some difficulty in... LRU'ing - thus the negative dentry problem). If that's the case it can decide to migrate objects and return MIGRATED? > In any case, it would be interesting to see if the existing shrinker interface > could be used for this stuff. We already best-effort-reclaim objects, maybe we > could best-effort-migrate objects too? The problem of reclaiming is adjacent to > migrating, and we already have infrastructure for it... You mean best-effort-migrate objects for defragmentation via shrinker interface? -- Cheers, Harry / Hyeonggon