mm: shmem: skip swapcache for swapin of synchronous swap device

With fast swap devices (such as zram), swapin latency is crucial to
applications.  For shmem swapin, similar to anonymous memory swapin, we
can skip the swapcache operation to improve swapin latency.  Testing 1G
shmem sequential swapin without THP enabled, I observed approximately a 6%
performance improvement: (Note: I repeated 5 times and took the mean data
for each test)

w/o patch	w/ patch	changes
534.8ms		501ms		+6.3%

In addition, currently, we always split the large swap entry stored in the
shmem mapping during shmem large folio swapin, which is not perfect,
especially with a fast swap device.  We should swap in the whole large
folio instead of splitting the precious large folios to take advantage of
the large folios and improve the swapin latency if the swap device is
synchronous device, which is similar to anonymous memory mTHP swapin. 
Testing 1G shmem sequential swapin with 64K mTHP and 2M mTHP, I observed
obvious performance improvement:

mTHP=64K
w/o patch	w/ patch	changes
550.4ms		169.6ms		+69%

mTHP=2M
w/o patch	w/ patch	changes
542.8ms		126.8ms		+77%

Note that skipping swapcache requires attention to concurrent swapin
scenarios.  Fortunately the swapcache_prepare() and
shmem_add_to_page_cache() can help identify concurrent swapin and large
swap entry split scenarios, and return -EEXIST for retry.

[[email protected]: use IS_ENABLED(), tweak comment grammar]
Link: https://lkml.kernel.org/r/3d9f3bd3bc6ec953054baff5134f66feeaae7c1e.1736301701.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kairui Song <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Ryan Roberts <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Baolin Wang

mm/shmem.c

@@ -1967,6 +1967,65 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
 	return ERR_PTR(error);
 }
 
+static struct folio *shmem_swap_alloc_folio(struct inode *inode,
+		struct vm_area_struct *vma, pgoff_t index,
+		swp_entry_t entry, int order, gfp_t gfp)
+{
+	struct shmem_inode_info *info = SHMEM_I(inode);
+	struct folio *new;
+	void *shadow;
+	int nr_pages;
+
+	/*
+	 * We have arrived here because our zones are constrained, so don't
+	 * limit chance of success with further cpuset and node constraints.
+	 */
+	gfp &= ~GFP_CONSTRAINT_MASK;
+	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && order > 0) {
+		gfp_t huge_gfp = vma_thp_gfp_mask(vma);
+
+		gfp = limit_gfp_mask(huge_gfp, gfp);
+	}
+
+	new = shmem_alloc_folio(gfp, order, info, index);
+	if (!new)
+		return ERR_PTR(-ENOMEM);
+
+	nr_pages = folio_nr_pages(new);
+	if (mem_cgroup_swapin_charge_folio(new, vma ? vma->vm_mm : NULL,
+					   gfp, entry)) {
+		folio_put(new);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	/*
+	 * Prevent parallel swapin from proceeding with the swap cache flag.
+	 *
+	 * Of course there is another possible concurrent scenario as well,
+	 * that is to say, the swap cache flag of a large folio has already
+	 * been set by swapcache_prepare(), while another thread may have
+	 * already split the large swap entry stored in the shmem mapping.
+	 * In this case, shmem_add_to_page_cache() will help identify the
+	 * concurrent swapin and return -EEXIST.
+	 */
+	if (swapcache_prepare(entry, nr_pages)) {
+		folio_put(new);
+		return ERR_PTR(-EEXIST);
+	}
+
+	__folio_set_locked(new);
+	__folio_set_swapbacked(new);
+	new->swap = entry;
+
+	mem_cgroup_swapin_uncharge_swap(entry, nr_pages);
+	shadow = get_shadow_from_swap_cache(entry);
+	if (shadow)
+		workingset_refault(new, shadow);
+	folio_add_lru(new);
+	swap_read_folio(new, NULL);
+	return new;
+}
+
 /*
  * When a page is moved from swapcache to shmem filecache (either by the
  * usual swapin of shmem_get_folio_gfp(), or by the less common swapoff of
@@ -2070,7 +2129,8 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
 }
 
 static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
-					 struct folio *folio, swp_entry_t swap)
+					 struct folio *folio, swp_entry_t swap,
+					 bool skip_swapcache)
 {
 	struct address_space *mapping = inode->i_mapping;
 	swp_entry_t swapin_error;
@@ -2086,7 +2146,8 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
 
 	nr_pages = folio_nr_pages(folio);
 	folio_wait_writeback(folio);
-	delete_from_swap_cache(folio);
+	if (!skip_swapcache)
+		delete_from_swap_cache(folio);
 	/*
 	 * Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
 	 * won't be 0 when inode is released and thus trigger WARN_ON(i_blocks)
@@ -2190,6 +2251,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct swap_info_struct *si;
 	struct folio *folio = NULL;
+	bool skip_swapcache = false;
 	swp_entry_t swap;
 	int error, nr_pages;
 
@@ -2211,6 +2273,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	/* Look it up and read it in.. */
 	folio = swap_cache_get_folio(swap, NULL, 0);
 	if (!folio) {
+		int order = xa_get_order(&mapping->i_pages, index);
+		bool fallback_order0 = false;
 		int split_order;
 
 		/* Or update major stats only when swapin succeeds?? */
@@ -2220,6 +2284,33 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 			count_memcg_event_mm(fault_mm, PGMAJFAULT);
 		}
 
+		/*
+		 * If uffd is active for the vma, we need per-page fault
+		 * fidelity to maintain the uffd semantics, then fallback
+		 * to swapin order-0 folio, as well as for zswap case.
+		 */
+		if (order > 0 && ((vma && unlikely(userfaultfd_armed(vma))) ||
+				  !zswap_never_enabled()))
+			fallback_order0 = true;
+
+		/* Skip swapcache for synchronous device. */
+		if (!fallback_order0 && data_race(si->flags & SWP_SYNCHRONOUS_IO)) {
+			folio = shmem_swap_alloc_folio(inode, vma, index, swap, order, gfp);
+			if (!IS_ERR(folio)) {
+				skip_swapcache = true;
+				goto alloced;
+			}
+
+			/*
+			 * Fallback to swapin order-0 folio unless the swap entry
+			 * already exists.
+			 */
+			error = PTR_ERR(folio);
+			folio = NULL;
+			if (error == -EEXIST)
+				goto failed;
+		}
+
 		/*
 		 * Now swap device can only swap in order 0 folio, then we
 		 * should split the large swap entry stored in the pagecache
@@ -2250,9 +2341,10 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 		}
 	}
 
+alloced:
 	/* We have to do this with folio locked to prevent races */
 	folio_lock(folio);
-	if (!folio_test_swapcache(folio) ||
+	if ((!skip_swapcache && !folio_test_swapcache(folio)) ||
 	    folio->swap.val != swap.val ||
 	    !shmem_confirm_swap(mapping, index, swap)) {
 		error = -EEXIST;
@@ -2288,7 +2380,12 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	if (sgp == SGP_WRITE)
 		folio_mark_accessed(folio);
 
-	delete_from_swap_cache(folio);
+	if (skip_swapcache) {
+		folio->swap.val = 0;
+		swapcache_clear(si, swap, nr_pages);
+	} else {
+		delete_from_swap_cache(folio);
+	}
 	folio_mark_dirty(folio);
 	swap_free_nr(swap, nr_pages);
 	put_swap_device(si);
@@ -2299,8 +2396,11 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 	if (!shmem_confirm_swap(mapping, index, swap))
 		error = -EEXIST;
 	if (error == -EIO)
-		shmem_set_folio_swapin_error(inode, index, folio, swap);
+		shmem_set_folio_swapin_error(inode, index, folio, swap,
+					     skip_swapcache);
 unlock:
+	if (skip_swapcache)
+		swapcache_clear(si, swap, folio_nr_pages(folio));
 	if (folio) {
 		folio_unlock(folio);
 		folio_put(folio);