On Mon, Nov 18, 2024 at 10:41 PM chenridong <chenridong@xxxxxxxxxx> wrote: > > > > On 2024/11/18 12:14, Barry Song wrote: > > On Mon, Nov 18, 2024 at 5:03 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > >> > >> On Sat, Nov 16, 2024 at 09:16:58AM +0000, Chen Ridong wrote: > >>> 2. In shrink_page_list function, if folioN is THP(2M), it may be splited > >>> and added to swap cache folio by folio. After adding to swap cache, > >>> it will submit io to writeback folio to swap, which is asynchronous. > >>> When shrink_page_list is finished, the isolated folios list will be > >>> moved back to the head of inactive lru. The inactive lru may just look > >>> like this, with 512 filioes have been move to the head of inactive lru. > >> > >> I was hoping that we'd be able to stop splitting the folio when adding > >> to the swap cache. Ideally. we'd add the whole 2MB and write it back > >> as a single unit. > > > > This is already the case: adding to the swapcache doesn’t require splitting > > THPs, but failing to allocate 2MB of contiguous swap slots will. > > > >> > >> This is going to become much more important with memdescs. We'd have to > >> allocate 512 struct folios to do this, which would be about 10 4kB pages, > >> and if we're trying to swap out memory, we're probably low on memory. > >> > >> So I don't like this solution you have at all because it doesn't help us > >> get to the solution we're going to need in about a year's time. > >> > > > > Ridong might need to clarify why this splitting is occurring. If it’s due to the > > failure to allocate swap slots, we still need a solution to address it. > > > > Thanks > > Barry > > shrink_folio_list > add_to_swap > folio_alloc_swap > get_swap_pages > scan_swap_map_slots > /* > * Swapfile is not block device or not using clusters so unable > * to allocate large entries. > */ > if (!(si->flags & SWP_BLKDEV) || !si->cluster_info) > return 0; > > In my test, I use a file as swap, which is not 'SWP_BLKDEV'. So it > failed to get get_swap_pages. Alright, a proper non-rotating swap block device would be much better. In your case, though, cluster allocation isn’t supported. > > I think this is a race issue between 'shrink_folio_list' executing and > writing back asynchronously. In my test, 512 folios(THP split) were > added to swap, only about 60 folios had not been written back when > 'move_folios_to_lru' was invoked after 'shrink_folio_list'. What if > writing back faster? Maybe this will happen even 32 folios(without THP) > are in the 'folio_list' of shrink_folio_list's inputs. On a real non-rotate swap device, the race condition would occur only when contiguous 2MB swap slots are unavailable. Hi Chris, I recall you mentioned unifying the code for swap devices and swap files, or for non-rotating and rotating devices. I assume a swap file (not a block device) would also be a practical user case? > > Best regards, > Ridong Thanks Barry
