Re: [PATCH v3 0/2] generic_file_buffered_read() refactoring & optimization
From: Kent Overstreet
Date: Fri Jun 19 2020 - 14:44:54 EST
On Fri, Jun 19, 2020 at 05:59:20AM -0700, Christoph Hellwig wrote:
> After looking at v2 yesterday I noticed I few things in the structure
> that I really didn't like:
>
> - using a struct page * return value just to communicate status codes
> - the extremely long function names
> - a generally somewhat non-intuitive split of the helpers
>
> I then hacked on top of it for a bit while sitting in a telephone
> conference. Below is my result, which passes a quick xfstests run.
> Note that this includes the refactoring and the batch lookup changes
> as I did it on top of your series:
I like it - I can't get your patch to apply to anything though, do you have it
up in a git repo anywhere?
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index f0ae9a6308cb4d..9e0aecd99950f4 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1972,273 +1972,360 @@ static void shrink_readahead_size_eio(struct file_ra_state *ra)
> ra->ra_pages /= 4;
> }
>
> -/**
> - * generic_file_buffered_read - generic file read routine
> - * @iocb: the iocb to read
> - * @iter: data destination
> - * @written: already copied
> - *
> - * This is a generic file read routine, and uses the
> - * mapping->a_ops->readpage() function for the actual low-level stuff.
> - *
> - * This is really ugly. But the goto's actually try to clarify some
> - * of the logic when it comes to error handling etc.
> - *
> - * Return:
> - * * total number of bytes copied, including those the were already @written
> - * * negative error code if nothing was copied
> - */
> -ssize_t generic_file_buffered_read(struct kiocb *iocb,
> - struct iov_iter *iter, ssize_t written)
> +static inline pgoff_t filemap_last_index(struct kiocb *iocb,
> + struct iov_iter *iter)
> {
> - struct file *filp = iocb->ki_filp;
> - struct address_space *mapping = filp->f_mapping;
> - struct inode *inode = mapping->host;
> - struct file_ra_state *ra = &filp->f_ra;
> - loff_t *ppos = &iocb->ki_pos;
> - pgoff_t index;
> - pgoff_t last_index;
> - pgoff_t prev_index;
> - unsigned long offset; /* offset into pagecache page */
> - unsigned int prev_offset;
> - int error = 0;
> -
> - if (unlikely(*ppos >= inode->i_sb->s_maxbytes))
> - return 0;
> - iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
> -
> - index = *ppos >> PAGE_SHIFT;
> - prev_index = ra->prev_pos >> PAGE_SHIFT;
> - prev_offset = ra->prev_pos & (PAGE_SIZE-1);
> - last_index = (*ppos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
> - offset = *ppos & ~PAGE_MASK;
> + return (iocb->ki_pos + iter->count + PAGE_SIZE - 1) >> PAGE_SHIFT;
> +}
>
> - for (;;) {
> - struct page *page;
> - pgoff_t end_index;
> - loff_t isize;
> - unsigned long nr, ret;
> +static inline unsigned long filemap_nr_pages(struct kiocb *iocb,
> + struct iov_iter *iter)
> +{
> + return filemap_last_index(iocb, iter) - (iocb->ki_pos >> PAGE_SHIFT);
> +}
>
> - cond_resched();
> -find_page:
> - if (fatal_signal_pending(current)) {
> - error = -EINTR;
> - goto out;
> - }
> +static int __filemap_read_not_uptodate(struct file *file, struct page *page)
> +{
> + int error;
>
> - page = find_get_page(mapping, index);
> - if (!page) {
> - if (iocb->ki_flags & IOCB_NOWAIT)
> - goto would_block;
> - page_cache_sync_readahead(mapping,
> - ra, filp,
> - index, last_index - index);
> - page = find_get_page(mapping, index);
> - if (unlikely(page == NULL))
> - goto no_cached_page;
> - }
> - if (PageReadahead(page)) {
> - page_cache_async_readahead(mapping,
> - ra, filp, page,
> - index, last_index - index);
> - }
> - if (!PageUptodate(page)) {
> - if (iocb->ki_flags & IOCB_NOWAIT) {
> - put_page(page);
> - goto would_block;
> - }
> + error = lock_page_killable(page);
> + if (error)
> + return error;
>
> + if (!PageUptodate(page)) {
> + if (!page->mapping) {
> /*
> - * See comment in do_read_cache_page on why
> - * wait_on_page_locked is used to avoid unnecessarily
> - * serialisations and why it's safe.
> + * invalidate_mapping_pages got it
> */
> - error = wait_on_page_locked_killable(page);
> - if (unlikely(error))
> - goto readpage_error;
> - if (PageUptodate(page))
> - goto page_ok;
> -
> - if (inode->i_blkbits == PAGE_SHIFT ||
> - !mapping->a_ops->is_partially_uptodate)
> - goto page_not_up_to_date;
> - /* pipes can't handle partially uptodate pages */
> - if (unlikely(iov_iter_is_pipe(iter)))
> - goto page_not_up_to_date;
> - if (!trylock_page(page))
> - goto page_not_up_to_date;
> - /* Did it get truncated before we got the lock? */
> - if (!page->mapping)
> - goto page_not_up_to_date_locked;
> - if (!mapping->a_ops->is_partially_uptodate(page,
> - offset, iter->count))
> - goto page_not_up_to_date_locked;
> - unlock_page(page);
> + error = AOP_TRUNCATED_PAGE;
> + } else {
> + error = -EIO;
> }
> -page_ok:
> - /*
> - * i_size must be checked after we know the page is Uptodate.
> - *
> - * Checking i_size after the check allows us to calculate
> - * the correct value for "nr", which means the zero-filled
> - * part of the page is not copied back to userspace (unless
> - * another truncate extends the file - this is desired though).
> - */
> + }
>
> - isize = i_size_read(inode);
> - end_index = (isize - 1) >> PAGE_SHIFT;
> - if (unlikely(!isize || index > end_index)) {
> - put_page(page);
> - goto out;
> - }
> + unlock_page(page);
> + if (error == -EIO)
> + shrink_readahead_size_eio(&file->f_ra);
> + return error;
> +}
>
> - /* nr is the maximum number of bytes to copy from this page */
> - nr = PAGE_SIZE;
> - if (index == end_index) {
> - nr = ((isize - 1) & ~PAGE_MASK) + 1;
> - if (nr <= offset) {
> - put_page(page);
> - goto out;
> - }
> - }
> - nr = nr - offset;
> +static int __filemap_read_one_page(struct file *file, struct page *page)
> +{
> + int error;
>
> - /* If users can be writing to this page using arbitrary
> - * virtual addresses, take care about potential aliasing
> - * before reading the page on the kernel side.
> - */
> - if (mapping_writably_mapped(mapping))
> - flush_dcache_page(page);
> + /*
> + * A previous I/O error may have been due to temporary failures, e.g.
> + * multipath errors. PG_error will be set again if readpage fails.
> + */
> + ClearPageError(page);
>
> - /*
> - * When a sequential read accesses a page several times,
> - * only mark it as accessed the first time.
> - */
> - if (prev_index != index || offset != prev_offset)
> - mark_page_accessed(page);
> - prev_index = index;
> + /*
> + * Start the actual read. The read will unlock the page.
> + */
> + error = file->f_mapping->a_ops->readpage(file, page);
> + if (!error && !PageUptodate(page))
> + return __filemap_read_not_uptodate(file, page);
> + return error;
> +}
>
> - /*
> - * Ok, we have the page, and it's up-to-date, so
> - * now we can copy it to user space...
> - */
> +static int filemap_read_one_page(struct kiocb *iocb, struct iov_iter *iter,
> + struct page *page, pgoff_t index)
> +{
> + struct file *file = iocb->ki_filp;
> + struct address_space *mapping = file->f_mapping;
> + loff_t pos = max(iocb->ki_pos, (loff_t)index << PAGE_SHIFT);
> + loff_t count = iocb->ki_pos + iter->count - pos;
> + int error;
>
> - ret = copy_page_to_iter(page, offset, nr, iter);
> - offset += ret;
> - index += offset >> PAGE_SHIFT;
> - offset &= ~PAGE_MASK;
> - prev_offset = offset;
> + /*
> + * See comment in do_read_cache_page on why wait_on_page_locked is used
> + * to avoid unnecessarily serialisations and why it's safe.
> + */
> + error = wait_on_page_locked_killable(page);
> + if (unlikely(error))
> + goto out_put_page;
>
> - put_page(page);
> - written += ret;
> - if (!iov_iter_count(iter))
> - goto out;
> - if (ret < nr) {
> - error = -EFAULT;
> - goto out;
> - }
> - continue;
> + if (PageUptodate(page))
> + return 0;
> +
> + if (mapping->host->i_blkbits == PAGE_SHIFT ||
> + !mapping->a_ops->is_partially_uptodate)
> + goto page_not_up_to_date;
> + /* pipes can't handle partially uptodate pages */
> + if (unlikely(iov_iter_is_pipe(iter)))
> + goto page_not_up_to_date;
> + if (!trylock_page(page))
> + goto page_not_up_to_date;
> + /* Did it get truncated before we got the lock? */
> + if (!page->mapping)
> + goto page_not_up_to_date_locked;
> + if (!mapping->a_ops->is_partially_uptodate(page, pos & ~PAGE_MASK,
> + count))
> + goto page_not_up_to_date_locked;
> +done:
> + unlock_page(page);
> + return 0;
>
> page_not_up_to_date:
> - /* Get exclusive access to the page ... */
> - error = lock_page_killable(page);
> - if (unlikely(error))
> - goto readpage_error;
> + /* Get exclusive access to the page ... */
> + error = lock_page_killable(page);
> + if (unlikely(error))
> + goto out_put_page;
>
> page_not_up_to_date_locked:
> - /* Did it get truncated before we got the lock? */
> - if (!page->mapping) {
> - unlock_page(page);
> - put_page(page);
> + /* Did it get truncated before we got the lock? */
> + if (!page->mapping) {
> + error = AOP_TRUNCATED_PAGE;
> + unlock_page(page);
> + goto out_put_page;
> + }
> +
> + /* Did somebody else fill it already? */
> + if (PageUptodate(page))
> + goto done;
> +
> + error = __filemap_read_one_page(file, page);
> + if (error)
> + goto out_put_page;
> + return 0;
> +
> +out_put_page:
> + put_page(page);
> + return error;
> +}
> +
> +static int filemap_read_get_pages(struct kiocb *iocb, struct iov_iter *iter,
> + struct page **pages, unsigned long max_pages)
> +{
> + struct file *file = iocb->ki_filp;
> + struct address_space *mapping = file->f_mapping;
> + unsigned long nr = min(filemap_nr_pages(iocb, iter), max_pages);
> + pgoff_t index = iocb->ki_pos >> PAGE_SHIFT;
> + int ret;
> +
> + if (fatal_signal_pending(current))
> + return -EINTR;
> +
> + ret = find_get_pages_contig(mapping, index, nr, pages);
> + if (ret)
> + return ret;
> +
> + if (iocb->ki_flags & IOCB_NOWAIT)
> + return -EAGAIN;
> +
> + page_cache_sync_readahead(mapping, &file->f_ra, file, index,
> + filemap_nr_pages(iocb, iter));
> +
> + ret = find_get_pages_contig(mapping, index, nr, pages);
> + if (ret)
> + return ret;
> +
> + /*
> + * Ok, it wasn't cached, so we need to create a new page..
> + */
> + pages[0] = page_cache_alloc(mapping);
> + if (!pages[0])
> + return -ENOMEM;
> +
> + ret = add_to_page_cache_lru(pages[0], mapping, index,
> + mapping_gfp_constraint(mapping, GFP_KERNEL));
> + if (ret) {
> + if (ret == -EEXIST)
> + ret = 0;
> + goto out_put_page;
> + }
> +
> + ret = __filemap_read_one_page(file, pages[0]);
> + if (ret) {
> + if (ret == AOP_TRUNCATED_PAGE)
> + ret = 0;
> + goto out_put_page;
> + }
> +
> + return 1;
> +
> +out_put_page:
> + put_page(pages[0]);
> + return ret;
> +}
> +
> +static int filemap_read_pages(struct kiocb *iocb, struct iov_iter *iter,
> + struct page **pages, unsigned int nr_pages)
> +{
> + struct file *file = iocb->ki_filp;
> + struct address_space *mapping = file->f_mapping;
> + pgoff_t last_index = filemap_last_index(iocb, iter);
> + pgoff_t index = iocb->ki_pos >> PAGE_SHIFT;
> + unsigned int cur_page, j;
> + int err = 0;
> +
> + for (cur_page = 0; cur_page < nr_pages; cur_page++, index++) {
> + struct page *page = pages[cur_page];
> +
> + if (PageReadahead(page))
> + page_cache_async_readahead(mapping, &file->f_ra, file,
> + page, index, last_index - index);
> +
> + if (PageUptodate(page))
> continue;
> - }
>
> - /* Did somebody else fill it already? */
> - if (PageUptodate(page)) {
> - unlock_page(page);
> - goto page_ok;
> + if (iocb->ki_flags & IOCB_NOWAIT) {
> + put_page(page);
> + err = -EAGAIN;
> + goto out_put_pages;
> }
>
> -readpage:
> - /*
> - * A previous I/O error may have been due to temporary
> - * failures, eg. multipath errors.
> - * PG_error will be set again if readpage fails.
> - */
> - ClearPageError(page);
> - /* Start the actual read. The read will unlock the page. */
> - error = mapping->a_ops->readpage(filp, page);
> + err = filemap_read_one_page(iocb, iter, page, index);
> + if (err)
> + goto out_put_pages;
> + }
>
> - if (unlikely(error)) {
> - if (error == AOP_TRUNCATED_PAGE) {
> - put_page(page);
> - error = 0;
> - goto find_page;
> - }
> - goto readpage_error;
> - }
> + return cur_page;
>
> - if (!PageUptodate(page)) {
> - error = lock_page_killable(page);
> - if (unlikely(error))
> - goto readpage_error;
> - if (!PageUptodate(page)) {
> - if (page->mapping == NULL) {
> - /*
> - * invalidate_mapping_pages got it
> - */
> - unlock_page(page);
> - put_page(page);
> - goto find_page;
> - }
> - unlock_page(page);
> - shrink_readahead_size_eio(ra);
> - error = -EIO;
> - goto readpage_error;
> - }
> - unlock_page(page);
> - }
> +out_put_pages:
> + for (j = cur_page + 1; j < nr_pages; j++)
> + put_page(pages[j]);
> + if (cur_page == 0) {
> + if (err == AOP_TRUNCATED_PAGE)
> + err = 0;
> + return err;
> + }
> + return cur_page;
> +}
>
> - goto page_ok;
> +static int filemap_do_read(struct kiocb *iocb, struct iov_iter *iter,
> + struct page **pages, unsigned long max_pages)
> +{
> + struct file *filp = iocb->ki_filp;
> + struct address_space *mapping = filp->f_mapping;
> + struct file_ra_state *ra = &filp->f_ra;
> + loff_t isize, end_offset;
> + int nr_pages, i;
> + bool writably_mapped;
>
> -readpage_error:
> - /* UHHUH! A synchronous read error occurred. Report it */
> - put_page(page);
> - goto out;
> + cond_resched();
> +
> +was_truncated:
> + nr_pages = filemap_read_get_pages(iocb, iter, pages, max_pages);
> + if (nr_pages > 0)
> + nr_pages = filemap_read_pages(iocb, iter, pages, nr_pages);
> + if (nr_pages == 0)
> + goto was_truncated;
> + if (nr_pages < 0)
> + return nr_pages;
> +
> + /*
> + * i_size must be checked after we know the pages are Uptodate.
> + *
> + * Checking i_size after the check allows us to calculate the correct
> + * value for "nr_pages", which means the zero-filled part of the page is
> + * not copied back to userspace (unless another truncate extends the
> + * file - this is desired though).
> + */
> + isize = i_size_read(mapping->host);
> + if (unlikely(iocb->ki_pos >= isize))
> + goto put_pages;
> +
> + end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count);
> +
> + while ((iocb->ki_pos >> PAGE_SHIFT) + nr_pages >
> + (end_offset + PAGE_SIZE - 1) >> PAGE_SHIFT)
> + put_page(pages[--nr_pages]);
> +
> + /*
> + * Once we start copying data, we don't want to be touching any
> + * cachelines that might be contended:
> + */
> + writably_mapped = mapping_writably_mapped(mapping);
> +
> + /*
> + * When a sequential read accesses a page several times, only mark it as
> + * accessed the first time.
> + */
> + if (iocb->ki_pos >> PAGE_SHIFT != ra->prev_pos >> PAGE_SHIFT)
> + mark_page_accessed(pages[0]);
> + for (i = 1; i < nr_pages; i++)
> + mark_page_accessed(pages[i]);
> +
> + for (i = 0; i < nr_pages; i++) {
> + unsigned offset = iocb->ki_pos & ~PAGE_MASK;
> + unsigned bytes = min_t(loff_t, end_offset - iocb->ki_pos,
> + PAGE_SIZE - offset);
> + unsigned copied;
>
> -no_cached_page:
> /*
> - * Ok, it wasn't cached, so we need to create a new
> - * page..
> + * If users can be writing to this page using arbitrary virtual
> + * addresses, take care about potential aliasing before reading
> + * the page on the kernel side.
> */
> - page = page_cache_alloc(mapping);
> - if (!page) {
> - error = -ENOMEM;
> - goto out;
> - }
> - error = add_to_page_cache_lru(page, mapping, index,
> - mapping_gfp_constraint(mapping, GFP_KERNEL));
> - if (error) {
> - put_page(page);
> - if (error == -EEXIST) {
> - error = 0;
> - goto find_page;
> - }
> - goto out;
> - }
> - goto readpage;
> + if (writably_mapped)
> + flush_dcache_page(pages[i]);
> +
> + copied = copy_page_to_iter(pages[i], offset, bytes, iter);
> +
> + iocb->ki_pos += copied;
> + ra->prev_pos = iocb->ki_pos;
> +
> + if (copied < bytes)
> + return -EFAULT;
> }
>
> -would_block:
> - error = -EAGAIN;
> -out:
> - ra->prev_pos = prev_index;
> - ra->prev_pos <<= PAGE_SHIFT;
> - ra->prev_pos |= prev_offset;
> +put_pages:
> + for (i = 0; i < nr_pages; i++)
> + put_page(pages[i]);
> + return 0;
> +}
> +
> +/**
> + * generic_file_buffered_read - generic file read routine
> + * @iocb: the iocb to read
> + * @iter: data destination
> + * @written: already copied
> + *
> + * This is a generic file read routine, and uses the
> + * mapping->a_ops->readpage() function for the actual low-level stuff.
> + *
> + * Return:
> + * * total number of bytes copied, including those the were already @written
> + * * negative error code if nothing was copied
> + */
> +ssize_t generic_file_buffered_read(struct kiocb *iocb,
> + struct iov_iter *iter, ssize_t written)
> +{
> + struct address_space *mapping = iocb->ki_filp->f_mapping;
> + size_t orig_count = iov_iter_count(iter);
> + struct inode *inode = mapping->host;
> + struct page *page_array[8], **pages = NULL;
> + unsigned max_pages = filemap_nr_pages(iocb, iter);
> + int error;
>
> - *ppos = ((loff_t)index << PAGE_SHIFT) + offset;
> - file_accessed(filp);
> - return written ? written : error;
> + if (unlikely(iocb->ki_pos >= inode->i_sb->s_maxbytes))
> + return 0;
> + iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
> +
> + if (max_pages > ARRAY_SIZE(page_array))
> + pages = kmalloc_array(max_pages, sizeof(void *), GFP_KERNEL);
> +
> + if (!pages) {
> + pages = page_array;
> + max_pages = ARRAY_SIZE(page_array);
> + }
> +
> + do {
> + error = filemap_do_read(iocb, iter, pages, max_pages);
> + if (error)
> + break;
> + } while (iov_iter_count(iter) && iocb->ki_pos < i_size_read(inode));
> +
> + file_accessed(iocb->ki_filp);
> + written += orig_count - iov_iter_count(iter);
> +
> + if (pages != page_array)
> + kfree(pages);
> +
> + if (unlikely(!written))
> + return error;
> + return written;
> }
> EXPORT_SYMBOL_GPL(generic_file_buffered_read);
>