On 2024/3/26 07:50, Yosry Ahmed wrote: > After the rbtree to xarray conversion, and dropping zswap_entry.refcount > and zswap_entry.value, the only members of zswap_entry utilized by > zero-filled pages are zswap_entry.length (always 0) and > zswap_entry.objcg. Store the objcg pointer directly in the xarray as a > tagged pointer and avoid allocating a zswap_entry completely for > zero-filled pages. > > This simplifies the code as we no longer need to special case > zero-length cases. We are also able to further separate the zero-filled > pages handling logic and completely isolate them within store/load > helpers. Handling tagged xarray pointers is handled in these two > helpers, as well as the newly introduced helper for freeing tree > elements, zswap_tree_free_element(). > > There is also a small performance improvement observed over 50 runs of > kernel build test (kernbench) comparing the mean build time on a skylake > machine when building the kernel in a cgroup v1 container with a 3G > limit. This is on top of the improvement from dropping support for > non-zero same-filled pages: > > base patched % diff > real 69.915 69.757 -0.229% > user 2956.147 2955.244 -0.031% > sys 2594.718 2575.747 -0.731% > > This probably comes from avoiding the zswap_entry allocation and > cleanup/freeing for zero-filled pages. Note that the percentage of > zero-filled pages during this test was only around 1.5% on average. > Practical workloads could have a larger proportion of such pages (e.g. > Johannes observed around 10% [1]), so the performance improvement should > be larger. > > This change also saves a small amount of memory due to less allocated > zswap_entry's. In the kernel build test above, we save around 2M of > slab usage when we swap out 3G to zswap. > > [1]https://lore.kernel.org/linux-mm/20240320210716.GH294822@xxxxxxxxxxx/ > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> The code looks good, just one comment below. Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx> > --- > mm/zswap.c | 137 ++++++++++++++++++++++++++++++----------------------- > 1 file changed, 78 insertions(+), 59 deletions(-) > > diff --git a/mm/zswap.c b/mm/zswap.c > index 413d9242cf500..efc323bab2f22 100644 > --- a/mm/zswap.c > +++ b/mm/zswap.c > @@ -183,12 +183,11 @@ static struct shrinker *zswap_shrinker; > * struct zswap_entry > * [..] > > @@ -1531,26 +1552,27 @@ bool zswap_load(struct folio *folio) > struct page *page = &folio->page; > struct xarray *tree = swap_zswap_tree(swp); > struct zswap_entry *entry; > + struct obj_cgroup *objcg; > + void *elem; > > VM_WARN_ON_ONCE(!folio_test_locked(folio)); > > - entry = xa_erase(tree, offset); > - if (!entry) > + elem = xa_erase(tree, offset); > + if (!elem) > return false; > > - if (entry->length) > + if (!zswap_load_zero_filled(elem, page, &objcg)) { > + entry = elem; nit: entry seems no use anymore. > + objcg = entry->objcg; > zswap_decompress(entry, page); > - else > - clear_highpage(page); > + } > > count_vm_event(ZSWPIN); > - if (entry->objcg) > - count_objcg_event(entry->objcg, ZSWPIN); > - > - zswap_entry_free(entry); > + if (objcg) > + count_objcg_event(objcg, ZSWPIN); > > + zswap_tree_free_element(elem); > folio_mark_dirty(folio); > - > return true; > } [..]
