On 21/10/2024 22:09, Yosry Ahmed wrote: > On Fri, Oct 18, 2024 at 3:50 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote: >> >> As mentioned in [1], there is a significant improvement in no >> readahead swapin performance for super fast devices when skipping >> swapcache. > > FYI, Kairui was working on removing the swapcache bypass completely, > which I think may be a good thing: > https://lore.kernel.org/lkml/20240326185032.72159-1-ryncsn@xxxxxxxxx/ > > However, that series is old, since before the large folio swapin > support, so I am not sure if/when he intends to refresh it. > > In his approach there is still a swapin path for synchronous swapin > though, which we can still utilize for zswap. > >> >> With large folio zswapin support added in later patches, this will also >> mean this path will also act as "readahead" by swapping in multiple >> pages into large folios. further improving performance. >> >> [1] https://lore.kernel.org/all/1505886205-9671-5-git-send-email-minchan@xxxxxxxxxx/T/#m5a792a04dfea20eb7af4c355d00503efe1c86a93 >> >> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx> >> --- >> include/linux/zswap.h | 6 ++++++ >> mm/memory.c | 3 ++- >> mm/page_io.c | 1 - >> mm/zswap.c | 46 +++++++++++++++++++++++++++++++++++++++++++ >> 4 files changed, 54 insertions(+), 2 deletions(-) >> >> diff --git a/include/linux/zswap.h b/include/linux/zswap.h >> index d961ead91bf1..e418d75db738 100644 >> --- a/include/linux/zswap.h >> +++ b/include/linux/zswap.h >> @@ -27,6 +27,7 @@ struct zswap_lruvec_state { >> unsigned long zswap_total_pages(void); >> bool zswap_store(struct folio *folio); >> bool zswap_load(struct folio *folio); >> +bool zswap_present_test(swp_entry_t swp, int nr_pages); >> void zswap_invalidate(swp_entry_t swp); >> int zswap_swapon(int type, unsigned long nr_pages); >> void zswap_swapoff(int type); >> @@ -49,6 +50,11 @@ static inline bool zswap_load(struct folio *folio) >> return false; >> } >> >> +static inline bool zswap_present_test(swp_entry_t swp, int nr_pages) >> +{ >> + return false; >> +} >> + >> static inline void zswap_invalidate(swp_entry_t swp) {} >> static inline int zswap_swapon(int type, unsigned long nr_pages) >> { >> diff --git a/mm/memory.c b/mm/memory.c >> index 03e5452dd0c0..49d243131169 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -4289,7 +4289,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> swapcache = folio; >> >> if (!folio) { >> - if (data_race(si->flags & SWP_SYNCHRONOUS_IO) && >> + if ((data_race(si->flags & SWP_SYNCHRONOUS_IO) || >> + zswap_present_test(entry, 1)) && >> __swap_count(entry) == 1) { >> /* skip swapcache */ >> folio = alloc_swap_folio(vmf); >> diff --git a/mm/page_io.c b/mm/page_io.c >> index 4aa34862676f..2a15b197968a 100644 >> --- a/mm/page_io.c >> +++ b/mm/page_io.c >> @@ -602,7 +602,6 @@ void swap_read_folio(struct folio *folio, struct swap_iocb **plug) >> unsigned long pflags; >> bool in_thrashing; >> >> - VM_BUG_ON_FOLIO(!folio_test_swapcache(folio) && !synchronous, folio); >> VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio); >> VM_BUG_ON_FOLIO(folio_test_uptodate(folio), folio); >> >> diff --git a/mm/zswap.c b/mm/zswap.c >> index 7f00cc918e7c..f4b03071b2fb 100644 >> --- a/mm/zswap.c >> +++ b/mm/zswap.c >> @@ -1576,6 +1576,52 @@ bool zswap_store(struct folio *folio) >> return ret; >> } >> >> +static bool swp_offset_in_zswap(unsigned int type, pgoff_t offset) >> +{ >> + return (offset >> SWAP_ADDRESS_SPACE_SHIFT) < nr_zswap_trees[type]; > > I am not sure I understand what we are looking for here. When does > this return false? Aren't the zswap trees always allocated during > swapon? > Hi Yosry, Thanks for the review! It becomes useful in patch 3 when trying to determine if a large folio can be allocated. For e.g. if the swap entry is the last entry of the last tree, and 1M folios are enabled (nr_pages = 256), then the while loop in zswap_present_test will try to access a tree that doesn't exist from the 2nd 4K page onwards if we dont have this check in zswap_present_test. >> +} >> + >> +/* Returns true if the entire folio is in zswap */ > > There isn't really a folio at this point, maybe "Returns true if the > entire range is in zswap"? Will change, Thanks! > > Also, this is racy because an exclusive load, invalidation, or > writeback can cause an entry to be removed from zswap. Under what > conditions is this safe? The caller can probably guarantee we don't > race against invalidation, but can we guarantee that concurrent > exclusive loads or writebacks don't happen? > > If the answer is yes, this needs to be properly documented. swapcache_prepare should stop things from becoming racy. lets say trying to swapin a mTHP of size 32 pages: - T1 is doing do_swap_page, T2 is doing zswap_writeback. - T1 - Check if the entire 32 pages is in zswap, swapcache_prepare(entry, nr_pages) in do_swap_page is not yet called. - T2 - zswap_writeback_entry starts and lets say writes page 2 to swap. it calls __read_swap_cache_async -> swapcache_prepare increments swap_map count, writes page to swap. - T1 - swapcache_prepare is then called and fails and then there will be a pagefault again for it. I will try and document this better. > >> +bool zswap_present_test(swp_entry_t swp, int nr_pages) >> +{ >> + pgoff_t offset = swp_offset(swp), tree_max_idx; >> + int max_idx = 0, i = 0, tree_offset = 0; >> + unsigned int type = swp_type(swp); >> + struct zswap_entry *entry = NULL; >> + struct xarray *tree; >> + >> + while (i < nr_pages) { >> + tree_offset = offset + i; >> + /* Check if the tree exists. */ >> + if (!swp_offset_in_zswap(type, tree_offset)) >> + return false; >> + >> + tree = swap_zswap_tree(swp_entry(type, tree_offset)); >> + XA_STATE(xas, tree, tree_offset); > > Please do not mix declarations with code. > >> + >> + tree_max_idx = tree_offset % SWAP_ADDRESS_SPACE_PAGES ? >> + ALIGN(tree_offset, SWAP_ADDRESS_SPACE_PAGES) : >> + ALIGN(tree_offset + 1, SWAP_ADDRESS_SPACE_PAGES); > > Does this work if we always use ALIGN(tree_offset + 1, > SWAP_ADDRESS_SPACE_PAGES)? Yes, I think max_idx = min(offset + nr_pages, ALIGN(tree_offset + 1, SWAP_ADDRESS_SPACE_PAGES)) - 1; will work. I will test it out, Thanks! > >> + max_idx = min(offset + nr_pages, tree_max_idx) - 1; >> + rcu_read_lock(); >> + xas_for_each(&xas, entry, max_idx) { >> + if (xas_retry(&xas, entry)) >> + continue; >> + i++; >> + } >> + rcu_read_unlock(); >> + /* >> + * If xas_for_each exits because entry is NULL and > > nit: add () to the end of function names (i.e. xas_for_each()) > >> + * the number of entries checked are less then max idx, > > s/then/than > >> + * then zswap does not contain the entire folio. >> + */ >> + if (!entry && offset + i <= max_idx) >> + return false; >> + } >> + >> + return true; >> +} >> + >> bool zswap_load(struct folio *folio) >> { >> swp_entry_t swp = folio->swap; >> -- >> 2.43.5 >>
