[PATCH v3 3/7] hfsplus: take the bitmap page lock for allocate/free

[Viacheslav Dubeyko]

The hfsplus_block_allocate() and hfsplus_block_free() kmap
the allocation bitmap's pages and modify their bits in place
under sbi->alloc_mutex, but without holding the page lock.
That leaves the read-modify-write of the bitmap bits
unprotected against a concurrent writeback of the same page,
which can read a partially-updated bitmap word or race with
the dirty-bit update.

Signed-off-by: Viacheslav Dubeyko <slava@xxxxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxx>
cc: John Paul Adrian Glaubitz <glaubitz@xxxxxxxxxxxxxxxxxxx>
cc: Yangtao Li <frank.li@xxxxxxxx>
cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
v2
Christoph Hellwig has detected that taking the page lock around
the kmap/modify/kunmap section is not enough on its own:
writeback drops the page lock before the write actually completes,
so a mutator that only waits on the lock can still start rewriting
a page whose old contents are still in flight to the device.
Mark the allocation file's mapping with mapping_set_stable_writes()
and call folio_wait_stable() right after taking the page lock in
both functions, so a mutator also waits out any writeback that was
already in progress when it acquired the lock.

v3
Matthew Wilcox recommended to use folio_wait_writeback()
instead of folio_wait_stable().
---
 fs/hfsplus/bitmap.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/fs/hfsplus/bitmap.c b/fs/hfsplus/bitmap.c
index 1b3af8c87cad..30178ea47362 100644
--- a/fs/hfsplus/bitmap.c
+++ b/fs/hfsplus/bitmap.c
@@ -39,6 +39,8 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 		start = size;
 		goto out;
 	}
+	lock_page(page);
+	folio_wait_writeback(page_folio(page));
 	pptr = kmap_local_page(page);
 	curr = pptr + (offset & (PAGE_CACHE_BITS - 1)) / 32;
 	i = offset % 32;
@@ -75,6 +77,7 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 			curr++;
 		}
 		kunmap_local(pptr);
+		unlock_page(page);
 		offset += PAGE_CACHE_BITS;
 		if (offset >= size)
 			break;
@@ -84,6 +87,8 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 			start = size;
 			goto out;
 		}
+		lock_page(page);
+		folio_wait_writeback(page_folio(page));
 		curr = pptr = kmap_local_page(page);
 		if ((size ^ offset) / PAGE_CACHE_BITS)
 			end = pptr + PAGE_CACHE_BITS / 32;
@@ -98,6 +103,9 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 	start = offset + (curr - pptr) * 32 + i;
 	if (start >= size) {
 		hfs_dbg("bitmap full\n");
+		kunmap_local(pptr);
+		unlock_page(page);
+		start = size;
 		goto out;
 	}
 	/* do any partial u32 at the start */
@@ -128,6 +136,7 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 		}
 		set_page_dirty(page);
 		kunmap_local(pptr);
+		unlock_page(page);
 		offset += PAGE_CACHE_BITS;
 		page = read_mapping_page(mapping, offset / PAGE_CACHE_BITS,
 					 NULL);
@@ -135,6 +144,8 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 			start = size;
 			goto out;
 		}
+		lock_page(page);
+		folio_wait_writeback(page_folio(page));
 		pptr = kmap_local_page(page);
 		curr = pptr;
 		end = pptr + PAGE_CACHE_BITS / 32;
@@ -152,6 +163,7 @@ int hfsplus_block_allocate(struct super_block *sb, u32 size,
 	*curr = cpu_to_be32(n);
 	set_page_dirty(page);
 	kunmap_local(pptr);
+	unlock_page(page);
 	*max = offset + (curr - pptr) * 32 + i - start;
 	sbi->free_blocks -= *max;
 	hfsplus_mark_mdb_dirty(sb);
@@ -185,6 +197,8 @@ int hfsplus_block_free(struct super_block *sb, u32 offset, u32 count)
 	page = read_mapping_page(mapping, pnr, NULL);
 	if (IS_ERR(page))
 		goto kaboom;
+	lock_page(page);
+	folio_wait_writeback(page_folio(page));
 	pptr = kmap_local_page(page);
 	curr = pptr + (offset & (PAGE_CACHE_BITS - 1)) / 32;
 	end = pptr + PAGE_CACHE_BITS / 32;
@@ -216,9 +230,12 @@ int hfsplus_block_free(struct super_block *sb, u32 offset, u32 count)
 			break;
 		set_page_dirty(page);
 		kunmap_local(pptr);
+		unlock_page(page);
 		page = read_mapping_page(mapping, ++pnr, NULL);
 		if (IS_ERR(page))
 			goto kaboom;
+		lock_page(page);
+		folio_wait_writeback(page_folio(page));
 		pptr = kmap_local_page(page);
 		curr = pptr;
 		end = pptr + PAGE_CACHE_BITS / 32;
@@ -232,6 +249,7 @@ int hfsplus_block_free(struct super_block *sb, u32 offset, u32 count)
 out:
 	set_page_dirty(page);
 	kunmap_local(pptr);
+	unlock_page(page);
 	sbi->free_blocks += len;
 	hfsplus_mark_mdb_dirty(sb);
 	mutex_unlock(&sbi->alloc_mutex);
-- 
2.43.0