[PATCH 5/5] fs/buffer: reduce stack usage on bh_read_iter()

Luis Chamberlain
2024-12-18T02:26:31Z

Now that we can read asynchronously buffer heads from a folio in
chunks, we can chop up bh_read_iter() with a smaller array size.
Use an array of 8 to avoid stack growth warnings on systems with
huge base page sizes.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
fs/buffer.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index b8ba72f2f211..bfa9c09b8597 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2415,7 +2415,10 @@ static void bh_read_batch_async(struct folio *folio,
(__tmp); \
(__tmp) = bh_next(__tmp, __head))

+#define MAX_BUF_CHUNK 8
+
struct bh_iter {
+ int chunk_number;
sector_t iblock;
get_block_t *get_block;
bool any_get_block_error;
@@ -2424,7 +2427,7 @@ struct bh_iter {
};

/*
- * Reads up to MAX_BUF_PER_PAGE buffer heads at a time on a folio on the given
+ * Reads up to MAX_BUF_CHUNK buffer heads at a time on a folio on the given
* block range iblock to lblock and helps update the number of buffer-heads
* which were not uptodate or unmapped for which we issued an async read for
* on iter->bh_folio_reads for the full folio. Returns the last buffer-head we
@@ -2436,10 +2439,11 @@ static struct buffer_head *bh_read_iter(struct folio *folio,
struct inode *inode,
struct bh_iter *iter, sector_t lblock)
{
- struct buffer_head *arr[MAX_BUF_PER_PAGE];
+ struct buffer_head *arr[MAX_BUF_CHUNK];
struct buffer_head *bh = pivot, *last;
int nr = 0, i = 0;
size_t blocksize = head->b_size;
+ int chunk_idx = MAX_BUF_CHUNK * iter->chunk_number;
bool no_reads = false;
bool fully_mapped = false;

@@ -2447,7 +2451,8 @@ static struct buffer_head *bh_read_iter(struct folio *folio,

/* collect buffers not uptodate and not mapped yet */
for_each_bh_pivot(bh, last, head) {
- BUG_ON(nr >= MAX_BUF_PER_PAGE);
+ if (nr >= MAX_BUF_CHUNK)
+ break;

if (buffer_uptodate(bh)) {
iter->iblock++;
@@ -2487,8 +2492,7 @@ static struct buffer_head *bh_read_iter(struct folio *folio,
}

iter->bh_folio_reads += nr;
-
- WARN_ON_ONCE(!bh_is_last(last, head));
+ iter->chunk_number++;

if (bh_is_last(last, head)) {
if (!iter->bh_folio_reads)
@@ -2518,6 +2522,7 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
struct buffer_head *bh, *head;
struct bh_iter iter = {
.get_block = get_block,
+ .chunk_number = 0,
.unmapped = 0,
.any_get_block_error = false,
.bh_folio_reads = 0,
--
2.43.0