On Sat 03-02-24 08:11:46, Christoph Hellwig wrote:
> 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-foli 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.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Looks good. Feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
> include/linux/writeback.h | 4 +
> mm/page-writeback.c | 192 ++++++++++++++++++++++----------------
> 2 files changed, 118 insertions(+), 78 deletions(-)
>
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index f67b3ea866a0fb..9845cb62e40b2d 100644
> --- a/include/linux/writeback.h
> +++ b/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_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);
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3abb053e70580e..5fe4cdb7dbd61a 100644
> --- a/mm/page-writeback.c
> +++ b/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 @@ 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.
> - *
> - * 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_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:
> folio_batch_release(&wbc->fbatch);
> if (wbc->range_cyclic)
> mapping->writeback_index = folio->index + folio_nr_pages(folio);
> + 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);
> --
> 2.39.2
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
