Re: [RFC 0/4] convert create_page_buffers to create_folio_buffers

Matthew Wilcox
2023-04-15T02:32:06Z

On Fri, Apr 14, 2023 at 06:01:16PM -0700, Luis Chamberlain wrote:
> a) dynamically allocate those now
> b) do a cursory review of the users of that and prepare them
> to grok buffer heads which are blocksize based rather than
> PAGE_SIZE based. So we just try to kill MAX_BUF_PER_PAGE.
>
> Without a) I think buffers after PAGE_SIZE won't get submit_bh() or lock for
> bs > PAGE_SIZE right now.

Worse, we'll overflow the array and corrupt the stack.

This one is a simple fix ...

+++ b/fs/buffer.c
@@ -2282,7 +2282,7 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
{
struct inode *inode = folio->mapping->host;
sector_t iblock, lblock;
- struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
+ struct buffer_head *bh, *head;
unsigned int blocksize, bbits;
int nr, i;
int fully_mapped = 1;
@@ -2335,7 +2335,6 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
if (buffer_uptodate(bh))
continue;
}
- arr[nr++] = bh;
} while (i++, iblock++, (bh = bh->b_this_page) != head);

if (fully_mapped)
@@ -2353,24 +2352,27 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
}

/* Stage two: lock the buffers */
- for (i = 0; i < nr; i++) {
- bh = arr[i];
+ bh = head;
+ do {
lock_buffer(bh);
mark_buffer_async_read(bh);
- }
+ bh = bh->b_this_page;
+ } while (bh != head);

/*
* Stage 3: start the IO. Check for uptodateness
* inside the buffer lock in case another process reading
* the underlying blockdev brought it uptodate (the sct fix).
*/
- for (i = 0; i < nr; i++) {
- bh = arr[i];
+ bh = head;
+ do {
if (buffer_uptodate(bh))
end_buffer_async_read(bh, 1);
else
submit_bh(REQ_OP_READ, bh);
- }
+ bh = bh->b_this_page;
+ } while (bh != head);
+
return 0;
}
EXPORT_SYMBOL(block_read_full_folio);