The patch titled
Subject: writeback: add a writeback iterator
has been added to the -mm mm-unstable branch. Its filename is
writeback-add-a-writeback-iterator.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/writeback-add-a-writeback-iterator.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Christoph Hellwig <hch@xxxxxx>
Subject: writeback: add a writeback iterator
Date: Thu, 15 Feb 2024 07:36:48 +0100
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@xxxxxx
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Dave Chinner <dchinner@xxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/writeback.h | 4
mm/page-writeback.c | 192 +++++++++++++++++++++---------------
2 files changed, 118 insertions(+), 78 deletions(-)
--- a/include/linux/writeback.h~writeback-add-a-writeback-iterator
+++ a/include/linux/writeback.h
@@ -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_flag
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);
--- a/mm/page-writeback.c~writeback-add-a-writeback-iterator
+++ a/mm/page-writeback.c
@@ -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 @@ retry:
}
/**
- * 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.
- *
- * 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 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.
+ * 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.
*
- * Return: %0 on success, negative error code otherwise
+ * 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.
+ *
+ * If there was an error in the per-folio writeback inside the writeback_iter()
+ * loop, @error should be set to the error value.
+ *
+ * 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 (wbc->range_cyclic) {
- wbc->index = mapping->writeback_index; /* prev offset */
- end = -1;
- } 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);
+ if (!folio) {
+ folio_batch_init(&wbc->fbatch);
+ wbc->saved_err = *error = 0;
- for (;;) {
- folio = writeback_get_folio(mapping, wbc);
- if (!folio)
- break;
+ /*
+ * 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;
- error = writepage(folio, wbc, data);
+ /*
+ * 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->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_spa
* 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);
_
Patches currently in -mm which might be from hch@xxxxxx are
writeback-dont-call-mapping_set_error-on-aop_writepage_activate.patch
writeback-fix-done_index-when-hitting-the-wbc-nr_to_write.patch
writeback-also-update-wbc-nr_to_write-on-writeback-failure.patch
writeback-only-update-writeback_index-for-range_cyclic-writeback.patch
writeback-rework-the-loop-termination-condition-in-write_cache_pages.patch
writeback-add-a-writeback-iterator.patch
