Re: [PATCH v7 06/24] mm: Rename various 'offset' parameters to 'index'

From: John Hubbard
Date: Thu Feb 20 2020 - 21:21:05 EST

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On 2/19/20 1:00 PM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
>
> The word 'offset' is used ambiguously to mean 'byte offset within
> a page', 'byte offset from the start of the file' and 'page offset
> from the start of the file'. Use 'index' to mean 'page offset
> from the start of the file' throughout the readahead code.

And: use 'count' to mean 'number of pages'. Since the patch also changes
req_size to req_count.

I'm casting about for a nice place in the code, to put your above note...and
there isn't one. But would it be terrible to just put a short comment
block at the top of this file, just so we have it somewhere? It's pretty
cool to settle on a consistent terminology, so backing it up with "yes, we
actually mean it" documentation would be even better.

One minor note below, but no bugs spotted, and this clarifies the code a bit, so:

Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>

thanks,
--
John Hubbard
NVIDIA

>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> mm/readahead.c | 86 ++++++++++++++++++++++++--------------------------
> 1 file changed, 42 insertions(+), 44 deletions(-)
>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 6a9d99229bd6..096cf9020648 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -156,7 +156,7 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
> * We really don't want to intermingle reads and writes like that.
> */
> void __do_page_cache_readahead(struct address_space *mapping,
> - struct file *filp, pgoff_t offset, unsigned long nr_to_read,
> + struct file *filp, pgoff_t index, unsigned long nr_to_read,
> unsigned long lookahead_size)
> {
> struct inode *inode = mapping->host;
> @@ -181,7 +181,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> * Preallocate as many pages as we will need.
> */
> for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
> - pgoff_t page_offset = offset + page_idx;
> + pgoff_t page_offset = index + page_idx;

This line seems to unrepentantly mix offset and index, still. :)

>
> if (page_offset > end_index)
> break;
> @@ -220,7 +220,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> * memory at once.
> */
> void force_page_cache_readahead(struct address_space *mapping,
> - struct file *filp, pgoff_t offset, unsigned long nr_to_read)
> + struct file *filp, pgoff_t index, unsigned long nr_to_read)
> {
> struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
> struct file_ra_state *ra = &filp->f_ra;
> @@ -240,9 +240,9 @@ void force_page_cache_readahead(struct address_space *mapping,
>
> if (this_chunk > nr_to_read)
> this_chunk = nr_to_read;
> - __do_page_cache_readahead(mapping, filp, offset, this_chunk, 0);
> + __do_page_cache_readahead(mapping, filp, index, this_chunk, 0);
>
> - offset += this_chunk;
> + index += this_chunk;
> nr_to_read -= this_chunk;
> }
> }
> @@ -323,21 +323,21 @@ static unsigned long get_next_ra_size(struct file_ra_state *ra,
> */
>
> /*
> - * Count contiguously cached pages from @offset-1 to @offset-@max,
> + * Count contiguously cached pages from @index-1 to @index-@max,
> * this count is a conservative estimation of
> * - length of the sequential read sequence, or
> * - thrashing threshold in memory tight systems
> */
> static pgoff_t count_history_pages(struct address_space *mapping,
> - pgoff_t offset, unsigned long max)
> + pgoff_t index, unsigned long max)
> {
> pgoff_t head;
>
> rcu_read_lock();
> - head = page_cache_prev_miss(mapping, offset - 1, max);
> + head = page_cache_prev_miss(mapping, index - 1, max);
> rcu_read_unlock();
>
> - return offset - 1 - head;
> + return index - 1 - head;
> }
>
> /*
> @@ -345,13 +345,13 @@ static pgoff_t count_history_pages(struct address_space *mapping,
> */
> static int try_context_readahead(struct address_space *mapping,
> struct file_ra_state *ra,
> - pgoff_t offset,
> + pgoff_t index,
> unsigned long req_size,
> unsigned long max)
> {
> pgoff_t size;
>
> - size = count_history_pages(mapping, offset, max);
> + size = count_history_pages(mapping, index, max);
>
> /*
> * not enough history pages:
> @@ -364,10 +364,10 @@ static int try_context_readahead(struct address_space *mapping,
> * starts from beginning of file:
> * it is a strong indication of long-run stream (or whole-file-read)
> */
> - if (size >= offset)
> + if (size >= index)
> size *= 2;
>
> - ra->start = offset;
> + ra->start = index;
> ra->size = min(size + req_size, max);
> ra->async_size = 1;
>
> @@ -379,13 +379,13 @@ static int try_context_readahead(struct address_space *mapping,
> */
> static void ondemand_readahead(struct address_space *mapping,
> struct file_ra_state *ra, struct file *filp,
> - bool hit_readahead_marker, pgoff_t offset,
> + bool hit_readahead_marker, pgoff_t index,
> unsigned long req_size)
> {
> struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
> unsigned long max_pages = ra->ra_pages;
> unsigned long add_pages;
> - pgoff_t prev_offset;
> + pgoff_t prev_index;
>
> /*
> * If the request exceeds the readahead window, allow the read to
> @@ -397,15 +397,15 @@ static void ondemand_readahead(struct address_space *mapping,
> /*
> * start of file
> */
> - if (!offset)
> + if (!index)
> goto initial_readahead;
>
> /*
> - * It's the expected callback offset, assume sequential access.
> + * It's the expected callback index, assume sequential access.
> * Ramp up sizes, and push forward the readahead window.
> */
> - if ((offset == (ra->start + ra->size - ra->async_size) ||
> - offset == (ra->start + ra->size))) {
> + if ((index == (ra->start + ra->size - ra->async_size) ||
> + index == (ra->start + ra->size))) {
> ra->start += ra->size;
> ra->size = get_next_ra_size(ra, max_pages);
> ra->async_size = ra->size;
> @@ -422,14 +422,14 @@ static void ondemand_readahead(struct address_space *mapping,
> pgoff_t start;
>
> rcu_read_lock();
> - start = page_cache_next_miss(mapping, offset + 1, max_pages);
> + start = page_cache_next_miss(mapping, index + 1, max_pages);
> rcu_read_unlock();
>
> - if (!start || start - offset > max_pages)
> + if (!start || start - index > max_pages)
> return;
>
> ra->start = start;
> - ra->size = start - offset; /* old async_size */
> + ra->size = start - index; /* old async_size */
> ra->size += req_size;
> ra->size = get_next_ra_size(ra, max_pages);
> ra->async_size = ra->size;
> @@ -444,29 +444,29 @@ static void ondemand_readahead(struct address_space *mapping,
>
> /*
> * sequential cache miss
> - * trivial case: (offset - prev_offset) == 1
> - * unaligned reads: (offset - prev_offset) == 0
> + * trivial case: (index - prev_index) == 1
> + * unaligned reads: (index - prev_index) == 0
> */
> - prev_offset = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
> - if (offset - prev_offset <= 1UL)
> + prev_index = (unsigned long long)ra->prev_pos >> PAGE_SHIFT;
> + if (index - prev_index <= 1UL)
> goto initial_readahead;
>
> /*
> * Query the page cache and look for the traces(cached history pages)
> * that a sequential stream would leave behind.
> */
> - if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
> + if (try_context_readahead(mapping, ra, index, req_size, max_pages))
> goto readit;
>
> /*
> * standalone, small random read
> * Read as is, and do not pollute the readahead state.
> */
> - __do_page_cache_readahead(mapping, filp, offset, req_size, 0);
> + __do_page_cache_readahead(mapping, filp, index, req_size, 0);
> return;
>
> initial_readahead:
> - ra->start = offset;
> + ra->start = index;
> ra->size = get_init_ra_size(req_size, max_pages);
> ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
>
> @@ -477,7 +477,7 @@ static void ondemand_readahead(struct address_space *mapping,
> * the resulted next readahead window into the current one.
> * Take care of maximum IO pages as above.
> */
> - if (offset == ra->start && ra->size == ra->async_size) {
> + if (index == ra->start && ra->size == ra->async_size) {
> add_pages = get_next_ra_size(ra, max_pages);
> if (ra->size + add_pages <= max_pages) {
> ra->async_size = add_pages;
> @@ -496,9 +496,8 @@ static void ondemand_readahead(struct address_space *mapping,
> * @mapping: address_space which holds the pagecache and I/O vectors
> * @ra: file_ra_state which holds the readahead state
> * @filp: passed on to ->readpage() and ->readpages()
> - * @offset: start offset into @mapping, in pagecache page-sized units
> - * @req_size: hint: total size of the read which the caller is performing in
> - * pagecache pages
> + * @index: Index of first page to be read.
> + * @req_count: Total number of pages being read by the caller.
> *
> * page_cache_sync_readahead() should be called when a cache miss happened:
> * it will submit the read. The readahead logic may decide to piggyback more
> @@ -507,7 +506,7 @@ static void ondemand_readahead(struct address_space *mapping,
> */
> void page_cache_sync_readahead(struct address_space *mapping,
> struct file_ra_state *ra, struct file *filp,
> - pgoff_t offset, unsigned long req_size)
> + pgoff_t index, unsigned long req_count)
> {
> /* no read-ahead */
> if (!ra->ra_pages)
> @@ -518,12 +517,12 @@ void page_cache_sync_readahead(struct address_space *mapping,
>
> /* be dumb */
> if (filp && (filp->f_mode & FMODE_RANDOM)) {
> - force_page_cache_readahead(mapping, filp, offset, req_size);
> + force_page_cache_readahead(mapping, filp, index, req_count);
> return;
> }
>
> /* do read-ahead */
> - ondemand_readahead(mapping, ra, filp, false, offset, req_size);
> + ondemand_readahead(mapping, ra, filp, false, index, req_count);
> }
> EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
>
> @@ -532,21 +531,20 @@ EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
> * @mapping: address_space which holds the pagecache and I/O vectors
> * @ra: file_ra_state which holds the readahead state
> * @filp: passed on to ->readpage() and ->readpages()
> - * @page: the page at @offset which has the PG_readahead flag set
> - * @offset: start offset into @mapping, in pagecache page-sized units
> - * @req_size: hint: total size of the read which the caller is performing in
> - * pagecache pages
> + * @page: The page at @index which triggered the readahead call.
> + * @index: Index of first page to be read.
> + * @req_count: Total number of pages being read by the caller.
> *
> * page_cache_async_readahead() should be called when a page is used which
> - * has the PG_readahead flag; this is a marker to suggest that the application
> + * is marked as PageReadahead; this is a marker to suggest that the application
> * has used up enough of the readahead window that we should start pulling in
> * more pages.
> */
> void
> page_cache_async_readahead(struct address_space *mapping,
> struct file_ra_state *ra, struct file *filp,
> - struct page *page, pgoff_t offset,
> - unsigned long req_size)
> + struct page *page, pgoff_t index,
> + unsigned long req_count)
> {
> /* no read-ahead */
> if (!ra->ra_pages)
> @@ -570,7 +568,7 @@ page_cache_async_readahead(struct address_space *mapping,
> return;
>
> /* do read-ahead */
> - ondemand_readahead(mapping, ra, filp, true, offset, req_size);
> + ondemand_readahead(mapping, ra, filp, true, index, req_count);
> }
> EXPORT_SYMBOL_GPL(page_cache_async_readahead);
>
>

Next message: Souza, Jose: "Re: [Intel-gfx] Linux 5.6-rc2"
Previous message: Anson Huang: "[PATCH] ARM: dts: imx: make wdog node name generic"
In reply to: Matthew Wilcox: "[PATCH v7 06/24] mm: Rename various 'offset' parameters to 'index'"
Next in thread: John Hubbard: "Re: [PATCH v7 06/24] mm: Rename various 'offset' parameters to 'index'"
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