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mm: don't cap request size based on read-ahead setting

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We ran into a funky issue, where someone doing 256K buffered reads saw
128K requests at the device level.  Turns out it is read-ahead capping
the request size, since we use 128K as the default setting.  This
doesn't make a lot of sense - if someone is issuing 256K reads, they
should see 256K reads, regardless of the read-ahead setting, if the
underlying device can support a 256K read in a single command.

This patch introduces a bdi hint, io_pages.  This is the soft max IO
size for the lower level, I've hooked it up to the bdev settings here.
Read-ahead is modified to issue the maximum of the user request size,
and the read-ahead max size, but capped to the max request size on the
device side.  The latter is done to avoid reading ahead too much, if the
application asks for a huge read.  With this patch, the kernel behaves
like the application expects.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Jens Axboe 2016-12-12 16:43:26 -08:00 • committed by Linus Torvalds
commit 9491ae4aad
4 changed files with 31 additions and 11 deletions
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39
mm/readahead.c
View file

@ -207,12 +207,21 @@ out:
* memory at once.
*/
int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
pgoff_t offset, unsigned long nr_to_read)
pgoff_t offset, unsigned long nr_to_read)
{
struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
struct file_ra_state *ra = &filp->f_ra;
unsigned long max_pages;
if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
return -EINVAL;
nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages);
/*
* If the request exceeds the readahead window, allow the read to
* be up to the optimal hardware IO size
*/
max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
nr_to_read = min(nr_to_read, max_pages);
while (nr_to_read) {
int err;
@ -369,9 +378,17 @@ ondemand_readahead(struct address_space *mapping,
bool hit_readahead_marker, pgoff_t offset,
unsigned long req_size)
{
unsigned long max = ra->ra_pages;
struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
unsigned long max_pages = ra->ra_pages;
pgoff_t prev_offset;
/*
* If the request exceeds the readahead window, allow the read to
* be up to the optimal hardware IO size
*/
if (req_size > max_pages && bdi->io_pages > max_pages)
max_pages = min(req_size, bdi->io_pages);
/*
* start of file
*/
@ -385,7 +402,7 @@ ondemand_readahead(struct address_space *mapping,
if ((offset == (ra->start + ra->size - ra->async_size) ||
offset == (ra->start + ra->size))) {
ra->start += ra->size;
ra->size = get_next_ra_size(ra, max);
ra->size = get_next_ra_size(ra, max_pages);
ra->async_size = ra->size;
goto readit;
}
@ -400,16 +417,16 @@ ondemand_readahead(struct address_space *mapping,
pgoff_t start;
rcu_read_lock();
start = page_cache_next_hole(mapping, offset + 1, max);
start = page_cache_next_hole(mapping, offset + 1, max_pages);
rcu_read_unlock();
if (!start || start - offset > max)
if (!start || start - offset > max_pages)
return 0;
ra->start = start;
ra->size = start - offset; /* old async_size */
ra->size += req_size;
ra->size = get_next_ra_size(ra, max);
ra->size = get_next_ra_size(ra, max_pages);
ra->async_size = ra->size;
goto readit;
}
@ -417,7 +434,7 @@ ondemand_readahead(struct address_space *mapping,
/*
* oversize read
*/
if (req_size > max)
if (req_size > max_pages)
goto initial_readahead;
/*
@ -433,7 +450,7 @@ ondemand_readahead(struct address_space *mapping,
* 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))
if (try_context_readahead(mapping, ra, offset, req_size, max_pages))
goto readit;
/*
@ -444,7 +461,7 @@ ondemand_readahead(struct address_space *mapping,
initial_readahead:
ra->start = offset;
ra->size = get_init_ra_size(req_size, max);
ra->size = get_init_ra_size(req_size, max_pages);
ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
readit:
@ -454,7 +471,7 @@ readit:
* the resulted next readahead window into the current one.
*/
if (offset == ra->start && ra->size == ra->async_size) {
ra->async_size = get_next_ra_size(ra, max);
ra->async_size = get_next_ra_size(ra, max_pages);
ra->size += ra->async_size;
}