> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Sent: Monday, January 6, 2025 5:17 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> hannes@xxxxxxxxxxx; nphamcs@xxxxxxxxx; chengming.zhou@xxxxxxxxx;
> usamaarif642@xxxxxxxxx; ryan.roberts@xxxxxxx; 21cnbao@xxxxxxxxx;
> akpm@xxxxxxxxxxxxxxxxxxxx; linux-crypto@xxxxxxxxxxxxxxx;
> herbert@xxxxxxxxxxxxxxxxxxx; davem@xxxxxxxxxxxxx;
> clabbe@xxxxxxxxxxxx; ardb@xxxxxxxxxx; ebiggers@xxxxxxxxxx;
> surenb@xxxxxxxxxx; Accardi, Kristen C <kristen.c.accardi@xxxxxxxxx>;
> Feghali, Wajdi K <wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh
> <vinodh.gopal@xxxxxxxxx>
> Subject: Re: [PATCH v5 11/12] mm: zswap: Restructure & simplify
> zswap_store() to make it amenable for batching.
>
> On Fri, Dec 20, 2024 at 10:31 PM Kanchana P Sridhar
> <kanchana.p.sridhar@xxxxxxxxx> wrote:
> >
> > This patch introduces zswap_store_folio() that implements all the computes
> > done earlier in zswap_store_page() for a single-page, for all the pages in
> > a folio. This allows us to move the loop over the folio's pages from
> > zswap_store() to zswap_store_folio().
> >
> > A distinct zswap_compress_folio() is also added, that simply calls
> > zswap_compress() for each page in the folio it is called with.
>
> The git diff looks funky, it may make things clearer to introduce
> zswap_compress_folio() in a separate patch.
Ok, will do so.
>
> >
> > zswap_store_folio() starts by allocating all zswap entries required to
> > store the folio. Next, it calls zswap_compress_folio() and finally, adds
> > the entries to the xarray and LRU.
> >
> > The error handling and cleanup required for all failure scenarios that can
> > occur while storing a folio in zswap is now consolidated to a
> > "store_folio_failed" label in zswap_store_folio().
> >
> > These changes facilitate developing support for compress batching in
> > zswap_store_folio().
> >
> > Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@xxxxxxxxx>
> > ---
> > mm/zswap.c | 183 +++++++++++++++++++++++++++++++++-----------------
> ---
> > 1 file changed, 116 insertions(+), 67 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index 99cd78891fd0..1be0f1807bfc 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1467,77 +1467,129 @@ static void shrink_worker(struct work_struct
> *w)
> > * main API
> > **********************************/
> >
> > -static ssize_t zswap_store_page(struct page *page,
> > - struct obj_cgroup *objcg,
> > - struct zswap_pool *pool)
> > +static bool zswap_compress_folio(struct folio *folio,
> > + struct zswap_entry *entries[],
> > + struct zswap_pool *pool)
> > {
> > - swp_entry_t page_swpentry = page_swap_entry(page);
> > - struct zswap_entry *entry, *old;
> > + long index, nr_pages = folio_nr_pages(folio);
> >
> > - /* allocate entry */
> > - entry = zswap_entry_cache_alloc(GFP_KERNEL, page_to_nid(page));
> > - if (!entry) {
> > - zswap_reject_kmemcache_fail++;
> > - return -EINVAL;
> > + for (index = 0; index < nr_pages; ++index) {
> > + struct page *page = folio_page(folio, index);
> > +
> > + if (!zswap_compress(page, entries[index], pool))
> > + return false;
> > }
> >
> > - if (!zswap_compress(page, entry, pool))
> > - goto compress_failed;
> > + return true;
> > +}
> >
> > - old = xa_store(swap_zswap_tree(page_swpentry),
> > - swp_offset(page_swpentry),
> > - entry, GFP_KERNEL);
> > - if (xa_is_err(old)) {
> > - int err = xa_err(old);
> > +/*
> > + * Store all pages in a folio.
> > + *
> > + * The error handling from all failure points is consolidated to the
> > + * "store_folio_failed" label, based on the initialization of the zswap
> entries'
> > + * handles to ERR_PTR(-EINVAL) at allocation time, and the fact that the
> > + * entry's handle is subsequently modified only upon a successful
> zpool_malloc()
> > + * after the page is compressed.
> > + */
> > +static ssize_t zswap_store_folio(struct folio *folio,
> > + struct obj_cgroup *objcg,
> > + struct zswap_pool *pool)
> > +{
> > + long index, nr_pages = folio_nr_pages(folio);
> > + struct zswap_entry **entries = NULL;
> > + int node_id = folio_nid(folio);
> > + size_t compressed_bytes = 0;
> >
> > - WARN_ONCE(err != -ENOMEM, "unexpected xarray error: %d\n",
> err);
> > - zswap_reject_alloc_fail++;
> > - goto store_failed;
> > + entries = kmalloc(nr_pages * sizeof(*entries), GFP_KERNEL);
>
> We can probably use kcalloc() here.
I am a little worried about the latency penalty of kcalloc() in the reclaim path,
especially since I am not relying on zero-initialized memory for "entries"..
>
> > + if (!entries)
> > + return -ENOMEM;
> > +
> > + /* allocate entries */
>
> This comment can be dropped.
Sure.
>
> > + for (index = 0; index < nr_pages; ++index) {
> > + entries[index] = zswap_entry_cache_alloc(GFP_KERNEL,
> node_id);
> > +
> > + if (!entries[index]) {
> > + zswap_reject_kmemcache_fail++;
> > + nr_pages = index;
> > + goto store_folio_failed;
> > + }
> > +
> > + entries[index]->handle = (unsigned long)ERR_PTR(-EINVAL);
> > }
> >
> > - /*
> > - * We may have had an existing entry that became stale when
> > - * the folio was redirtied and now the new version is being
> > - * swapped out. Get rid of the old.
> > - */
> > - if (old)
> > - zswap_entry_free(old);
> > + if (!zswap_compress_folio(folio, entries, pool))
> > + goto store_folio_failed;
> >
> > - /*
> > - * The entry is successfully compressed and stored in the tree, there is
> > - * no further possibility of failure. Grab refs to the pool and objcg.
> > - * These refs will be dropped by zswap_entry_free() when the entry is
> > - * removed from the tree.
> > - */
> > - zswap_pool_get(pool);
> > - if (objcg)
> > - obj_cgroup_get(objcg);
> > + for (index = 0; index < nr_pages; ++index) {
> > + swp_entry_t page_swpentry = page_swap_entry(folio_page(folio,
> index));
> > + struct zswap_entry *old, *entry = entries[index];
> > +
> > + old = xa_store(swap_zswap_tree(page_swpentry),
> > + swp_offset(page_swpentry),
> > + entry, GFP_KERNEL);
> > + if (xa_is_err(old)) {
> > + int err = xa_err(old);
> > +
> > + WARN_ONCE(err != -ENOMEM, "unexpected xarray error:
> %d\n", err);
> > + zswap_reject_alloc_fail++;
> > + goto store_folio_failed;
> > + }
> >
> > - /*
> > - * We finish initializing the entry while it's already in xarray.
> > - * This is safe because:
> > - *
> > - * 1. Concurrent stores and invalidations are excluded by folio lock.
> > - *
> > - * 2. Writeback is excluded by the entry not being on the LRU yet.
> > - * The publishing order matters to prevent writeback from seeing
> > - * an incoherent entry.
> > - */
> > - entry->pool = pool;
> > - entry->swpentry = page_swpentry;
> > - entry->objcg = objcg;
> > - entry->referenced = true;
> > - if (entry->length) {
> > - INIT_LIST_HEAD(&entry->lru);
> > - zswap_lru_add(&zswap_list_lru, entry);
> > + /*
> > + * We may have had an existing entry that became stale when
> > + * the folio was redirtied and now the new version is being
> > + * swapped out. Get rid of the old.
> > + */
> > + if (old)
> > + zswap_entry_free(old);
> > +
> > + /*
> > + * The entry is successfully compressed and stored in the tree,
> there is
> > + * no further possibility of failure. Grab refs to the pool and objcg.
> > + * These refs will be dropped by zswap_entry_free() when the
> entry is
> > + * removed from the tree.
> > + */
> > + zswap_pool_get(pool);
> > + if (objcg)
> > + obj_cgroup_get(objcg);
> > +
> > + /*
> > + * We finish initializing the entry while it's already in xarray.
> > + * This is safe because:
> > + *
> > + * 1. Concurrent stores and invalidations are excluded by folio
> lock.
> > + *
> > + * 2. Writeback is excluded by the entry not being on the LRU yet.
> > + * The publishing order matters to prevent writeback from seeing
> > + * an incoherent entry.
> > + */
> > + entry->pool = pool;
> > + entry->swpentry = page_swpentry;
> > + entry->objcg = objcg;
> > + entry->referenced = true;
> > + if (entry->length) {
> > + INIT_LIST_HEAD(&entry->lru);
> > + zswap_lru_add(&zswap_list_lru, entry);
> > + }
> > +
> > + compressed_bytes += entry->length;
> > }
> >
> > - return entry->length;
> > + kfree(entries);
> > +
> > + return compressed_bytes;
> > +
> > +store_folio_failed:
> > + for (index = 0; index < nr_pages; ++index) {
> > + if (!IS_ERR_VALUE(entries[index]->handle))
> > + zpool_free(pool->zpool, entries[index]->handle);
> > +
> > + zswap_entry_cache_free(entries[index]);
> > + }
>
> If there is a failure in xa_store() halfway through the entries, this
> loop will free all the compressed objects and entries. But, some of
> the entries are already in the xarray, and zswap_store() will try to
> free them again. This seems like a bug, or did I miss something here?
Thanks, great catch! Yes, this is a bug. I have a simple fix implemented,
that I am currently testing and will include in v6.
>
> > +
> > + kfree(entries);
> >
> > -store_failed:
> > - zpool_free(pool->zpool, entry->handle);
> > -compress_failed:
> > - zswap_entry_cache_free(entry);
> > return -EINVAL;
> > }
> >
> > @@ -1549,8 +1601,8 @@ bool zswap_store(struct folio *folio)
> > struct mem_cgroup *memcg = NULL;
> > struct zswap_pool *pool;
> > size_t compressed_bytes = 0;
> > + ssize_t bytes;
> > bool ret = false;
> > - long index;
> >
> > VM_WARN_ON_ONCE(!folio_test_locked(folio));
> > VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> > @@ -1584,15 +1636,11 @@ bool zswap_store(struct folio *folio)
> > mem_cgroup_put(memcg);
> > }
> >
> > - for (index = 0; index < nr_pages; ++index) {
> > - struct page *page = folio_page(folio, index);
> > - ssize_t bytes;
> > + bytes = zswap_store_folio(folio, objcg, pool);
> > + if (bytes < 0)
> > + goto put_pool;
> >
> > - bytes = zswap_store_page(page, objcg, pool);
> > - if (bytes < 0)
> > - goto put_pool;
> > - compressed_bytes += bytes;
> > - }
> > + compressed_bytes = bytes;
>
> What's the point of having both compressed_bytes and bytes now?
The main reason was to cleanly handle a negative error value returned in "bytes"
(declared as ssize_t), as against a true total "compressed_bytes" (declared as size_t)
for the folio to use for objcg charging. This is similar to the current mainline
code where zswap_store() calls zswap_store_page(). I was hoping to avoid potential
issues with overflow/underflow, and for maintainability. Let me know if this is Ok.
Thanks,
Kanchana
>
> >
> > if (objcg) {
> > obj_cgroup_charge_zswap(objcg, compressed_bytes);
> > @@ -1622,6 +1670,7 @@ bool zswap_store(struct folio *folio)
> > pgoff_t offset = swp_offset(swp);
> > struct zswap_entry *entry;
> > struct xarray *tree;
> > + long index;
> >
> > for (index = 0; index < nr_pages; ++index) {
> > tree = swap_zswap_tree(swp_entry(type, offset + index));
> > --
> > 2.27.0
> >
