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fs: Add IOCB_NOIO flag for generic_file_read_iter 

Add an IOCB_NOIO flag that indicates to generic_file_read_iter that it
shouldn't trigger any filesystem I/O for the actual request or for
readahead.  This allows to do tentative reads out of the page cache as
some filesystems allow, and to take the appropriate locks and retry the
reads only if the requested pages are not cached.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
This commit is contained in:
Andreas Gruenbacher 2019-11-21 23:25:07 +00:00
parent dcb7fd82c7
commit 41da51bce3
2 changed files with 22 additions and 2 deletions
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1
include/linux/fs.h
View File

@ -315,6 +315,7 @@ enum rw_hint {
#define IOCB_SYNC (1 << 5)
#define IOCB_WRITE (1 << 6)
#define IOCB_NOWAIT (1 << 7)
#define IOCB_NOIO (1 << 9)
struct kiocb {
struct file *ki_filp;

23
mm/filemap.c
View File

@ -2028,7 +2028,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
page = find_get_page(mapping, index);
if (!page) {
if (iocb->ki_flags & IOCB_NOWAIT)
if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_NOIO))
goto would_block;
page_cache_sync_readahead(mapping,
ra, filp,
@ -2038,6 +2038,10 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
goto no_cached_page;
}
if (PageReadahead(page)) {
if (iocb->ki_flags & IOCB_NOIO) {
put_page(page);
goto out;
}
page_cache_async_readahead(mapping,
ra, filp, page,
index, last_index - index);
@ -2160,6 +2164,11 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
}
readpage:
if (iocb->ki_flags & IOCB_NOIO) {
unlock_page(page);
put_page(page);
goto would_block;
}
/*
* A previous I/O error may have been due to temporary
* failures, eg. multipath errors.
@ -2249,9 +2258,19 @@ EXPORT_SYMBOL_GPL(generic_file_buffered_read);
*
* This is the "read_iter()" routine for all filesystems
* that can use the page cache directly.
*
* The IOCB_NOWAIT flag in iocb->ki_flags indicates that -EAGAIN shall
* be returned when no data can be read without waiting for I/O requests
* to complete; it doesn't prevent readahead.
*
* The IOCB_NOIO flag in iocb->ki_flags indicates that no new I/O
* requests shall be made for the read or for readahead. When no data
* can be read, -EAGAIN shall be returned. When readahead would be
* triggered, a partial, possibly empty read shall be returned.
*
* Return:
* * number of bytes copied, even for partial reads
* * negative error code if nothing was read
* * negative error code (or 0 if IOCB_NOIO) if nothing was read
*/
ssize_t
generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)