Re: [PATCH 19/19] writeback: simplify writeback iteration

Jan Kara
2024-01-30T10:46:09Z

On Thu 25-01-24 09:57:58, Christoph Hellwig wrote:
> Based on the feedback from Jan I've tried to figure out how to
> avoid the error magic in the for_each_writeback_folio. This patch
> tries to implement this by switching to an open while loop over a
> single writeback_iter() function:
>
> while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> ...
> }
>
> 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.
>
> Additionally it moves the AOP_WRITEPAGE_ACTIVATE out of the iterator
> and into the callers, in preparation for eventually killing it off
> with the phase out of write_cache_pages().
>
> To me this form of the loop feels easier to follow, and it has the
> added advantage that writeback_iter() can actually be nicely used in
> nested loops, which should help with further iterizing the iomap
> writeback code.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Looking at it now I'm thinking whether we would not be better off to
completely dump the 'error' argument of writeback_iter() /
writeback_iter_next() and just make all .writepage implementations set
wbc->err directly. But that means touching all the ~20 writepage
implementations we still have...

Couple of comments regarding this implementation below (overall I agree it
seems somewhat easier to follow the code).

> +/**
> + * writepage_iter - iterate folio of a mapping for writeback
> + * @mapping: address space structure to write
> + * @wbc: writeback context
> + * @folio: previously iterated folio (%NULL to start)
> + * @error: in-out pointer for writeback errors (see below)
> + *
> + * This function should be called in a while loop in the ->writepages
> + * implementation and returns the next folio for the writeback operation
> + * described by @wbc on @mapping.
> + *
> + * To start writeback @folio should be passed as NULL, for every following
> + * iteration the folio returned by this function previously should be passed.
> + * @error should contain the error from the previous writeback iteration when
> + * calling writeback_iter.
> + *
> + * 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.
> + */
> +struct folio *writeback_iter(struct address_space *mapping,
> + struct writeback_control *wbc, struct folio *folio, int *error)
> {
> - unsigned long nr = folio_nr_pages(folio);
> + if (folio) {
> + wbc->nr_to_write -= folio_nr_pages(folio);
> + if (*error && !wbc->err)
> + wbc->err = *error;
>
> - wbc->nr_to_write -= nr;
> -
> - /*
> - * Handle the legacy AOP_WRITEPAGE_ACTIVATE magic return value.
> - * Eventually all instances should just unlock the folio themselves and
> - * return 0;
> - */
> - if (error == AOP_WRITEPAGE_ACTIVATE) {
> - folio_unlock(folio);
> - error = 0;
> + /*
> + * For integrity sync we have to keep going until we have
> + * written all the folios we tagged for writeback prior to
> + * entering the writeback loop, even if we run past
> + * wbc->nr_to_write or encounter errors.
> + *
> + * This is because the file system may still have state to clear
> + * for each folio. We'll eventually return the first error
> + * encountered.
> + *
> + * For background writeback just push done_index past this folio
> + * so that we can just restart where we left off and media
> + * errors won't choke writeout for the entire file.
> + */
> + if (wbc->sync_mode == WB_SYNC_NONE &&
> + (wbc->err || wbc->nr_to_write <= 0))
> + goto finish;

I think it would be a bit more comprehensible if we replace the goto with:
folio_batch_release(&wbc->fbatch);
if (wbc->range_cyclic)
mapping->writeback_index =
folio->index + folio_nr_pages(folio);
*error = wbc->err;
return NULL;

> + } else {
> + if (wbc->range_cyclic)
> + wbc->index = mapping->writeback_index; /* prev offset */
> + else
> + wbc->index = wbc->range_start >> PAGE_SHIFT;
> + if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> + tag_pages_for_writeback(mapping, wbc->index,
> + wbc_end(wbc));
> + folio_batch_init(&wbc->fbatch);
> + wbc->err = 0;
> }
>
> - if (error && !wbc->err)
> - wbc->err = error;
> + folio = writeback_get_folio(mapping, wbc);
> + if (!folio)
> + goto finish;

And here we just need to do:
if (wbc->range_cyclic)
mapping->writeback_index = 0;
*error = wbc->err;
return NULL;

> + return folio;
> +
> +finish:
> + folio_batch_release(&wbc->fbatch);
>
> /*
> - * For integrity sync we have to keep going until we have written all
> - * the folios we tagged for writeback prior to entering the writeback
> - * loop, even if we run past wbc->nr_to_write or encounter errors.
> - * This is because the file system may still have state to clear for
> - * each folio. We'll eventually return the first error encountered.
> + * 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 background writeback just push done_index past this folio so that
> - * we can just restart where we left off and media errors won't choke
> - * writeout for the entire 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.
> */
> - if (wbc->sync_mode == WB_SYNC_NONE &&
> - (wbc->err || wbc->nr_to_write <= 0)) {
> - writeback_finish(mapping, wbc, folio->index + nr);
> - return NULL;
> + if (wbc->range_cyclic) {
> + WARN_ON_ONCE(wbc->sync_mode != WB_SYNC_NONE);
> + if (wbc->err || wbc->nr_to_write <= 0)
> + mapping->writeback_index =
> + folio->index + folio_nr_pages(folio);
> + else
> + mapping->writeback_index = 0;
> }
> -
> - return writeback_get_folio(mapping, wbc);
> + *error = wbc->err;
> + return NULL;
> }
>
> /**
> @@ -2563,13 +2575,17 @@ static int writeback_use_writepage(struct address_space *mapping,
> struct writeback_control *wbc)
> {
> struct blk_plug plug;
> - struct folio *folio;
> - int err;
> + struct folio *folio = 0;
^^ NULL please

> + int err = 0;
>
> blk_start_plug(&plug);
> - for_each_writeback_folio(mapping, wbc, folio, err) {
> + while ((folio = writeback_iter(mapping, wbc, folio, &err))) {
> err = mapping->a_ops->writepage(&folio->page, wbc);
> mapping_set_error(mapping, err);
> + if (err == AOP_WRITEPAGE_ACTIVATE) {
> + folio_unlock(folio);
> + err = 0;
> + }
> }
> blk_finish_plug(&plug);
>
> @@ -2590,6 +2606,8 @@ int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
> ret = mapping->a_ops->writepages(mapping, wbc);
> } else if (mapping->a_ops->writepage) {
> ret = writeback_use_writepage(mapping, wbc);
> + if (!ret)
> + ret = wbc->err;

AFAICT this should not be needed as writeback_iter() made sure wbc->err is
returned when set?

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR