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writeback: add a writeback iterator

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Refactor the code left in write_cache_pages into an iterator that the file
system can call to get the next folio for a writeback operation:

	struct folio *folio = NULL;

	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
		error = <do per-folio writeback>;
	}

The twist here is that the error value is passed by reference, so that the
iterator can restore it when breaking out of the loop.

Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
iterator and needs is just kept in the write_cache_pages legacy wrapper. 
in preparation for eventually killing it off.

Heavily based on a for_each* based iterator from Matthew Wilcox.

Link: https://lkml.kernel.org/r/20240215063649.2164017-14-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Christoph Hellwig 2024-02-15 07:36:48 +01:00 committed by Andrew Morton
parent a2cbc13638
commit cdc150b575
2 changed files with 116 additions and 76 deletions
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4
include/linux/writeback.h
View file

@ -82,6 +82,7 @@ struct writeback_control {
/* internal fields used by the ->writepages implementation: */
struct folio_batch fbatch;
pgoff_t index;
int saved_err;
#ifdef CONFIG_CGROUP_WRITEBACK
struct bdi_writeback *wb; /* wb this writeback is issued under */
@ -366,6 +367,9 @@ int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,
bool wb_over_bg_thresh(struct bdi_writeback *wb);
struct folio *writeback_iter(struct address_space *mapping,
struct writeback_control *wbc, struct folio *folio, int *error);
typedef int (*writepage_t)(struct folio *folio, struct writeback_control *wbc,
void *data);

188
mm/page-writeback.c
View file

@ -2325,18 +2325,18 @@ void __init page_writeback_init(void)
}
/**
* tag_pages_for_writeback - tag pages to be written by write_cache_pages
* tag_pages_for_writeback - tag pages to be written by writeback
* @mapping: address space structure to write
* @start: starting page index
* @end: ending page index (inclusive)
*
* This function scans the page range from @start to @end (inclusive) and tags
* all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
* that write_cache_pages (or whoever calls this function) will then use
* TOWRITE tag to identify pages eligible for writeback. This mechanism is
* used to avoid livelocking of writeback by a process steadily creating new
* dirty pages in the file (thus it is important for this function to be quick
* so that it can tag pages faster than a dirtying process can create them).
* all pages that have DIRTY tag set with a special TOWRITE tag. The caller
* can then use the TOWRITE tag to identify pages eligible for writeback.
* This mechanism is used to avoid livelocking of writeback by a process
* steadily creating new dirty pages in the file (thus it is important for this
* function to be quick so that it can tag pages faster than a dirtying process
* can create them).
*/
void tag_pages_for_writeback(struct address_space *mapping,
pgoff_t start, pgoff_t end)
@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
}
/**
* write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
* writeback_iter - iterate folio of a mapping for writeback
* @mapping: address space structure to write
* @wbc: subtract the number of written pages from *@wbc->nr_to_write
* @writepage: function called for each page
* @data: data passed to writepage function
* @wbc: writeback context
* @folio: previously iterated folio (%NULL to start)
* @error: in-out pointer for writeback errors (see below)
*
* If a page is already under I/O, write_cache_pages() skips it, even
* if it's dirty. This is desirable behaviour for memory-cleaning writeback,
* but it is INCORRECT for data-integrity system calls such as fsync(). fsync()
* and msync() need to guarantee that all the data which was dirty at the time
* the call was made get new I/O started against them. If wbc->sync_mode is
* WB_SYNC_ALL then we were called for data integrity and we must wait for
* existing IO to complete.
* This function returns the next folio for the writeback operation described by
* @wbc on @mapping and should be called in a while loop in the ->writepages
* implementation.
*
* To avoid livelocks (when other process dirties new pages), we first tag
* pages which should be written back with TOWRITE tag and only then start
* writing them. For data-integrity sync we have to be careful so that we do
* not miss some pages (e.g., because some other process has cleared TOWRITE
* tag we set). The rule we follow is that TOWRITE tag can be cleared only
* by the process clearing the DIRTY tag (and submitting the page for IO).
* To start the writeback operation, %NULL is passed in the @folio argument, and
* for every subsequent iteration the folio returned previously should be passed
* back in.
*
* To avoid deadlocks between range_cyclic writeback and callers that hold
* pages in PageWriteback to aggregate IO until write_cache_pages() returns,
* we do not loop back to the start of the file. Doing so causes a page
* lock/page writeback access order inversion - we should only ever lock
* multiple pages in ascending page->index order, and looping back to the start
* of the file violates that rule and causes deadlocks.
* If there was an error in the per-folio writeback inside the writeback_iter()
* loop, @error should be set to the error value.
*
* Return: %0 on success, negative error code otherwise
* Once the writeback described in @wbc has finished, this function will return
* %NULL and if there was an error in any iteration restore it to @error.
*
* Note: callers should not manually break out of the loop using break or goto
* but must keep calling writeback_iter() until it returns %NULL.
*
* Return: the folio to write or %NULL if the loop is done.
*/
int write_cache_pages(struct address_space *mapping,
struct writeback_control *wbc, writepage_t writepage,
void *data)
struct folio *writeback_iter(struct address_space *mapping,
struct writeback_control *wbc, struct folio *folio, int *error)
{
int ret = 0;
int error;
struct folio *folio;
pgoff_t end; /* Inclusive */
if (!folio) {
folio_batch_init(&wbc->fbatch);
wbc->saved_err = *error = 0;
if (wbc->range_cyclic) {
wbc->index = mapping->writeback_index; /* prev offset */
end = -1;
/*
* For range cyclic writeback we remember where we stopped so
* that we can continue where we stopped.
*
* For non-cyclic writeback we always start at the beginning of
* the passed in range.
*/
if (wbc->range_cyclic)
wbc->index = mapping->writeback_index;
else
wbc->index = wbc->range_start >> PAGE_SHIFT;
/*
* To avoid livelocks when other processes dirty new pages, we
* first tag pages which should be written back and only then
* start writing them.
*
* For data-integrity writeback we have to be careful so that we
* do not miss some pages (e.g., because some other process has
* cleared the TOWRITE tag we set). The rule we follow is that
* TOWRITE tag can be cleared only by the process clearing the
* DIRTY tag (and submitting the page for I/O).
*/
if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
tag_pages_for_writeback(mapping, wbc->index,
wbc_end(wbc));
} else {
wbc->index = wbc->range_start >> PAGE_SHIFT;
end = wbc->range_end >> PAGE_SHIFT;
}
if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
tag_pages_for_writeback(mapping, wbc->index, end);
folio_batch_init(&wbc->fbatch);
for (;;) {
folio = writeback_get_folio(mapping, wbc);
if (!folio)
break;
error = writepage(folio, wbc, data);
wbc->nr_to_write -= folio_nr_pages(folio);
if (error == AOP_WRITEPAGE_ACTIVATE) {
folio_unlock(folio);
error = 0;
}
WARN_ON_ONCE(*error > 0);
/*
* For integrity writeback we have to keep going until we have
@ -2510,33 +2509,70 @@ int write_cache_pages(struct address_space *mapping,
* wbc->nr_to_write or encounter the first error.
*/
if (wbc->sync_mode == WB_SYNC_ALL) {
if (error && !ret)
ret = error;
if (*error && !wbc->saved_err)
wbc->saved_err = *error;
} else {
if (error || wbc->nr_to_write <= 0)
if (*error || wbc->nr_to_write <= 0)
goto done;
}
}
/*
* For range cyclic writeback we need to remember where we stopped so
* that we can continue there next time we are called. If we hit the
* last page and there is more work to be done, wrap back to the start
* of the file.
*
* For non-cyclic writeback we always start looking up at the beginning
* of the file if we are called again, which can only happen due to
* -ENOMEM from the file system.
*/
folio_batch_release(&wbc->fbatch);
if (wbc->range_cyclic)
mapping->writeback_index = 0;
return ret;
folio = writeback_get_folio(mapping, wbc);
if (!folio) {
/*
* To avoid deadlocks between range_cyclic writeback and callers
* that hold pages in PageWriteback to aggregate I/O until
* the writeback iteration finishes, we do not loop back to the
* start of the file. Doing so causes a page lock/page
* writeback access order inversion - we should only ever lock
* multiple pages in ascending page->index order, and looping
* back to the start of the file violates that rule and causes
* deadlocks.
*/
if (wbc->range_cyclic)
mapping->writeback_index = 0;
/*
* Return the first error we encountered (if there was any) to
* the caller.
*/
*error = wbc->saved_err;
}
return folio;
done:
if (wbc->range_cyclic)
mapping->writeback_index = folio->index + folio_nr_pages(folio);
folio_batch_release(&wbc->fbatch);
return NULL;
}
/**
* write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
* @mapping: address space structure to write
* @wbc: subtract the number of written pages from *@wbc->nr_to_write
* @writepage: function called for each page
* @data: data passed to writepage function
*
* Return: %0 on success, negative error code otherwise
*
* Note: please use writeback_iter() instead.
*/
int write_cache_pages(struct address_space *mapping,
struct writeback_control *wbc, writepage_t writepage,
void *data)
{
struct folio *folio = NULL;
int error;
while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
error = writepage(folio, wbc, data);
if (error == AOP_WRITEPAGE_ACTIVATE) {
folio_unlock(folio);
error = 0;
}
}
return error;
}
EXPORT_SYMBOL(write_cache_pages);