[PATCH 1/5] fs: add read support for RWF_UNCACHED

Jens Axboe
2019-12-10T20:43:13Z

If RWF_UNCACHED is set for io_uring (or preadv2(2)), we'll drop the
cache for buffered reads if we are the ones instantiating it. If the
data is already cached, we leave it cached.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
include/linux/fs.h | 3 +++
include/uapi/linux/fs.h | 5 ++++-
mm/filemap.c | 46 ++++++++++++++++++++++++++++++++++++-----
3 files changed, 48 insertions(+), 6 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 98e0349adb52..092ea2a4319b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -314,6 +314,7 @@ enum rw_hint {
#define IOCB_SYNC (1 << 5)
#define IOCB_WRITE (1 << 6)
#define IOCB_NOWAIT (1 << 7)
+#define IOCB_UNCACHED (1 << 8)

struct kiocb {
struct file *ki_filp;
@@ -3418,6 +3419,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
if (flags & RWF_APPEND)
ki->ki_flags |= IOCB_APPEND;
+ if (flags & RWF_UNCACHED)
+ ki->ki_flags |= IOCB_UNCACHED;
return 0;
}

diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 379a612f8f1d..357ebb0e0c5d 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -299,8 +299,11 @@ typedef int __bitwise __kernel_rwf_t;
/* per-IO O_APPEND */
#define RWF_APPEND ((__force __kernel_rwf_t)0x00000010)

+/* drop cache after reading or writing data */
+#define RWF_UNCACHED ((__force __kernel_rwf_t)0x00000040)
+
/* mask of flags supported by the kernel */
#define RWF_SUPPORTED (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
- RWF_APPEND)
+ RWF_APPEND | RWF_UNCACHED)

#endif /* _UAPI_LINUX_FS_H */
diff --git a/mm/filemap.c b/mm/filemap.c
index bf6aa30be58d..ed23a11b3e34 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -933,8 +933,8 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
}
EXPORT_SYMBOL(add_to_page_cache_locked);

-int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
- pgoff_t offset, gfp_t gfp_mask)
+static int __add_to_page_cache(struct page *page, struct address_space *mapping,
+ pgoff_t offset, gfp_t gfp_mask, bool lru)
{
void *shadow = NULL;
int ret;
@@ -956,9 +956,17 @@ int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
WARN_ON_ONCE(PageActive(page));
if (!(gfp_mask & __GFP_WRITE) && shadow)
workingset_refault(page, shadow);
- lru_cache_add(page);
+ if (lru)
+ lru_cache_add(page);
}
return ret;
+
+}
+
+int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
+ pgoff_t offset, gfp_t gfp_mask)
+{
+ return __add_to_page_cache(page, mapping, offset, gfp_mask, true);
}
EXPORT_SYMBOL_GPL(add_to_page_cache_lru);

@@ -2032,6 +2040,7 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
offset = *ppos & ~PAGE_MASK;

for (;;) {
+ bool drop_page = false;
struct page *page;
pgoff_t end_index;
loff_t isize;
@@ -2048,6 +2057,9 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
if (!page) {
if (iocb->ki_flags & IOCB_NOWAIT)
goto would_block;
+ /* UNCACHED implies no read-ahead */
+ if (iocb->ki_flags & IOCB_UNCACHED)
+ goto no_cached_page;
page_cache_sync_readahead(mapping,
ra, filp,
index, last_index - index);
@@ -2147,6 +2159,26 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
offset &= ~PAGE_MASK;
prev_offset = offset;

+ /*
+ * If we're dropping this page due to drop-behind, then
+ * lock it first. Ignore errors here, we can just leave it
+ * in the page cache. Note that we didn't add this page to
+ * the LRU when we added it to the page cache. So if we
+ * fail removing it, or lock it, add to the LRU.
+ */
+ if (drop_page) {
+ bool addlru = true;
+
+ if (!lock_page_killable(page)) {
+ if (page->mapping == mapping)
+ addlru = !remove_mapping(mapping, page);
+ else
+ addlru = false;
+ unlock_page(page);
+ }
+ if (addlru)
+ lru_cache_add(page);
+ }
put_page(page);
written += ret;
if (!iov_iter_count(iter))
@@ -2234,8 +2266,12 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
error = -ENOMEM;
goto out;
}
- error = add_to_page_cache_lru(page, mapping, index,
- mapping_gfp_constraint(mapping, GFP_KERNEL));
+ if (iocb->ki_flags & IOCB_UNCACHED)
+ drop_page = true;
+
+ error = __add_to_page_cache(page, mapping, index,
+ mapping_gfp_constraint(mapping, GFP_KERNEL),
+ !drop_page);
if (error) {
put_page(page);
if (error == -EEXIST) {
--
2.24.0