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Message-ID: <5691442b-56c7-7b0d-d91b-275be52abb42@nvidia.com>
Date: Thu, 20 Feb 2020 19:19:58 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Matthew Wilcox <willy@...radead.org>,
<linux-fsdevel@...r.kernel.org>
CC: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
<linux-btrfs@...r.kernel.org>, <linux-erofs@...ts.ozlabs.org>,
<linux-ext4@...r.kernel.org>,
<linux-f2fs-devel@...ts.sourceforge.net>,
<cluster-devel@...hat.com>, <ocfs2-devel@....oracle.com>,
<linux-xfs@...r.kernel.org>
Subject: Re: [PATCH v7 09/24] mm: Put readahead pages in cache earlier
On 2/19/20 1:00 PM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
>
> When populating the page cache for readahead, mappings that use
> ->readpages must populate the page cache themselves as the pages are
> passed on a linked list which would normally be used for the page cache's
> LRU. For mappings that use ->readpage or the upcoming ->readahead method,
> we can put the pages into the page cache as soon as they're allocated,
> which solves a race between readahead and direct IO. It also lets us
> remove the gfp argument from read_pages().
>
> Use the new readahead_page() API to implement the repeated calls to
> ->readpage(), just like most filesystems will. This iterator also
> supports huge pages, even though none of the filesystems have been
> converted to use them yet.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
> include/linux/pagemap.h | 20 +++++++++++++++++
> mm/readahead.c | 48 +++++++++++++++++++++++++----------------
> 2 files changed, 49 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 55fcea0249e6..4989d330fada 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -647,8 +647,28 @@ struct readahead_control {
> /* private: use the readahead_* accessors instead */
> pgoff_t _index;
> unsigned int _nr_pages;
> + unsigned int _batch_count;
> };
>
> +static inline struct page *readahead_page(struct readahead_control *rac)
> +{
> + struct page *page;
> +
> + BUG_ON(rac->_batch_count > rac->_nr_pages);
> + rac->_nr_pages -= rac->_batch_count;
> + rac->_index += rac->_batch_count;
> + rac->_batch_count = 0;
Is it intentional, to set rac->_batch_count twice (here, and below)? The
only reason I can see is if a caller needs to use ->_batch_count in the
"return NULL" case, which doesn't seem to come up...
> +
> + if (!rac->_nr_pages)
> + return NULL;
> +
> + page = xa_load(&rac->mapping->i_pages, rac->_index);
> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> + rac->_batch_count = hpage_nr_pages(page);
> +
> + return page;
> +}
> +
> /* The number of pages in this readahead block */
> static inline unsigned int readahead_count(struct readahead_control *rac)
> {
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 83df5c061d33..aaa209559ba2 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -113,15 +113,14 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
>
> EXPORT_SYMBOL(read_cache_pages);
>
> -static void read_pages(struct readahead_control *rac, struct list_head *pages,
> - gfp_t gfp)
> +static void read_pages(struct readahead_control *rac, struct list_head *pages)
> {
> const struct address_space_operations *aops = rac->mapping->a_ops;
> + struct page *page;
> struct blk_plug plug;
> - unsigned page_idx;
>
> if (!readahead_count(rac))
> - return;
> + goto out;
>
> blk_start_plug(&plug);
>
> @@ -130,23 +129,23 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
> readahead_count(rac));
> /* Clean up the remaining pages */
> put_pages_list(pages);
> - goto out;
> - }
> -
> - for (page_idx = 0; page_idx < readahead_count(rac); page_idx++) {
> - struct page *page = lru_to_page(pages);
> - list_del(&page->lru);
> - if (!add_to_page_cache_lru(page, rac->mapping, page->index,
> - gfp))
> + rac->_index += rac->_nr_pages;
> + rac->_nr_pages = 0;
> + } else {
> + while ((page = readahead_page(rac))) {
> aops->readpage(rac->file, page);
> - put_page(page);
> + put_page(page);
> + }
> }
>
> -out:
> blk_finish_plug(&plug);
>
> BUG_ON(!list_empty(pages));
> - rac->_nr_pages = 0;
> + BUG_ON(readahead_count(rac));
> +
> +out:
> + /* If we were called due to a conflicting page, skip over it */
Tiny documentation nit: What if we were *not* called due to a conflicting page?
(And what is a "conflicting page", in this context, btw?) The next line unconditionally
moves the index ahead, so the "if" part of the comment really confuses me.
> + rac->_index++;
> }
>
> /*
> @@ -165,9 +164,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
> LIST_HEAD(page_pool);
> loff_t isize = i_size_read(inode);
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> + bool use_list = mapping->a_ops->readpages;
> struct readahead_control rac = {
> .mapping = mapping,
> .file = filp,
> + ._index = index,
> ._nr_pages = 0,
> };
> unsigned long i;
> @@ -184,6 +185,8 @@ void __do_page_cache_readahead(struct address_space *mapping,
> if (index + i > end_index)
> break;
>
> + BUG_ON(index + i != rac._index + rac._nr_pages);
> +
> page = xa_load(&mapping->i_pages, index + i);
> if (page && !xa_is_value(page)) {
> /*
> @@ -191,15 +194,22 @@ void __do_page_cache_readahead(struct address_space *mapping,
> * contiguous pages before continuing with the next
> * batch.
> */
> - read_pages(&rac, &page_pool, gfp_mask);
> + read_pages(&rac, &page_pool);
> continue;
> }
>
> page = __page_cache_alloc(gfp_mask);
> if (!page)
> break;
> - page->index = index + i;
> - list_add(&page->lru, &page_pool);
> + if (use_list) {
> + page->index = index + i;
> + list_add(&page->lru, &page_pool);
> + } else if (add_to_page_cache_lru(page, mapping, index + i,
> + gfp_mask) < 0) {
I still think you'll want to compare against !=0, rather than < 0, here.
> + put_page(page);
> + read_pages(&rac, &page_pool);
Doing a read_pages() in the error case is because...actually, I'm not sure yet.
Why do we do this? Effectively it's a retry?
> + continue;
> + }
> if (i == nr_to_read - lookahead_size)
> SetPageReadahead(page);
> rac._nr_pages++;
> @@ -210,7 +220,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> * uptodate then the caller will launch readpage again, and
> * will then handle the error.
> */
> - read_pages(&rac, &page_pool, gfp_mask);
> + read_pages(&rac, &page_pool);
> }
>
> /*
>
Didn't spot any actual errors, just mainly my own questions here. :)
thanks,
--
John Hubbard
NVIDIA
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