The VM_FAULT_RETRY case introduced a performance bug that leads to
excessive/unconditional mmap readarounds for wild random mmap reads.
A retried page fault means a mmap readahead miss(mmap_miss++) followed by
a hit(mmap_miss--) on the same page. This sticks mmap_miss, and thus stops
mmap readaround from being turned off for wild random reads. Fix it by an
extra mmap_miss increament in order to counteract the followed mmap hit.
Also make mmap_miss a more robust 'unsigned int', so that if ever mmap_miss
goes out of range, it only create _temporary_ performance impacts.
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
include/linux/fs.h | 2 +-
mm/filemap.c | 8 ++++++--
2 files changed, 7 insertions(+), 3 deletions(-)
--- mm.orig/mm/filemap.c
+++ mm/mm/filemap.c
@@ -1574,8 +1574,10 @@ retry_find:
vmf->pgoff, 1);
retry_ret = find_lock_page_retry(mapping, vmf->pgoff,
vma, &page, retry_flag);
- if (retry_ret == VM_FAULT_RETRY)
+ if (retry_ret == VM_FAULT_RETRY) {
+ ra->mmap_miss++; /* counteract the followed retry hit */
return retry_ret;
+ }
if (!page)
goto no_cached_page;
}
@@ -1617,8 +1619,10 @@ retry_find:
retry_find_retry:
retry_ret = find_lock_page_retry(mapping, vmf->pgoff,
vma, &page, retry_flag);
- if (retry_ret == VM_FAULT_RETRY)
+ if (retry_ret == VM_FAULT_RETRY) {
+ ra->mmap_miss++; /* counteract the followed retry hit */
return retry_ret;
+ }
if (!page)
goto no_cached_page;
}
--- mm.orig/include/linux/fs.h
+++ mm/include/linux/fs.h
@@ -824,7 +824,7 @@ struct file_ra_state {
there are only # of pages ahead */
unsigned int ra_pages; /* Maximum readahead window */
- int mmap_miss; /* Cache miss stat for mmap accesses */
+ unsigned int mmap_miss; /* Cache miss stat for mmap accesses */
loff_t prev_pos; /* Cache last read() position */
};
--
--
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