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Subject[PATCH 6/9] readahead: clean up and simplify the code for filemap page fault readahead
From: Linus Torvalds <torvalds@linux-foundation.org>

This shouldn't really change behavior all that much, but the single
rather complex function with read-ahead inside a loop etc is broken up
into more manageable pieces.

The behaviour is also less subtle, with the read-ahead being done up-front
rather than inside some subtle loop and thus avoiding the now unnecessary
extra state variables (ie "did_readaround" is gone).

Fengguang: the code split in fact fixed a bug reported by Pavel Levshin:
the PGMAJFAULT accounting used to be bypassed when MADV_RANDOM is set, in
which case the original code will directly jump to no_cached_page reading.

Cc: Pavel Levshin <lpk@581.spb.su>
Cc: wli@movementarian.org
Cc: Nick Piggin <npiggin@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
Ok, so this is something I did in Mexico when I wasn't scuba-diving, and
was "watching" the kids at the pool. It was brought on by looking at git
mmap file behaviour under cold-cache behaviour: git does ok, but my laptop
disk is really slow, and I tried to verify that the kernel did a
reasonable job of read-ahead when taking page faults.

I think it did, but quite frankly, the filemap_fault() code was totally
unreadable. So this separates out the read-ahead cases, and adds more
comments, and also changes it so that we do asynchronous read-ahead
*before* we actually wait for the page we are waiting for to become
unlocked.

Not that it seems to make any real difference on my laptop, but I really
hated how it was doing a

page = get_lock_page(..)

and then doing read-ahead after that: which just guarantees that we have
to wait for any out-standing IO on "page" to complete before we can even
submit any new read-ahead! That just seems totally broken!

So it replaces the "get_lock_page()" at the top with a broken-out page
cache lookup, which allows us to look at the page state flags and make
appropriate decisions on what we should do without waiting for the locked
bit to clear.

It does add many more lines than it removes:

mm/filemap.c | 192 +++++++++++++++++++++++++++++++++++++++-------------------
1 files changed, 130 insertions(+), 62 deletions(-)
but that's largely due to (a) the new function headers etc due to the
split-up and (b) new or extended comments especially about the helper
functions. The code, in many ways, is actually simpler, apart from the
fairly trivial expansion of the equivalent of "get_lock_page()" into the
function.

Comments? I tried to avoid changing the read-ahead logic itself, although
the old code did some strange things like doing *both* async readahead and
then looking up the page and doing sync readahead (which I think was just
due to the code being so damn messily organized, not on purpose).

Linus

---
mm/filemap.c | 156 +++++++++++++++++++++++++++----------------------
1 file changed, 89 insertions(+), 67 deletions(-)
--- mm.orig/mm/filemap.c
+++ mm/mm/filemap.c
@@ -1455,6 +1455,68 @@ static int page_cache_read(struct file *

#define MMAP_LOTSAMISS (100)

+/*
+ * Synchronous readahead happens when we don't even find
+ * a page in the page cache at all.
+ */
+static void do_sync_mmap_readahead(struct vm_area_struct *vma,
+ struct file_ra_state *ra,
+ struct file *file,
+ pgoff_t offset)
+{
+ unsigned long ra_pages;
+ struct address_space *mapping = file->f_mapping;
+
+ /* If we don't want any read-ahead, don't bother */
+ if (VM_RandomReadHint(vma))
+ return;
+
+ if (VM_SequentialReadHint(vma)) {
+ page_cache_sync_readahead(mapping, ra, file, offset, 1);
+ return;
+ }
+
+ if (ra->mmap_miss < INT_MAX)
+ ra->mmap_miss++;
+
+ /*
+ * Do we miss much more than hit in this file? If so,
+ * stop bothering with read-ahead. It will only hurt.
+ */
+ if (ra->mmap_miss > MMAP_LOTSAMISS)
+ return;
+
+ ra_pages = max_sane_readahead(ra->ra_pages);
+ if (ra_pages) {
+ pgoff_t start = 0;
+
+ if (offset > ra_pages / 2)
+ start = offset - ra_pages / 2;
+ do_page_cache_readahead(mapping, file, start, ra_pages);
+ }
+}
+
+/*
+ * Asynchronous readahead happens when we find the page and PG_readahead,
+ * so we want to possibly extend the readahead further..
+ */
+static void do_async_mmap_readahead(struct vm_area_struct *vma,
+ struct file_ra_state *ra,
+ struct file *file,
+ struct page *page,
+ pgoff_t offset)
+{
+ struct address_space *mapping = file->f_mapping;
+
+ /* If we don't want any read-ahead, don't bother */
+ if (VM_RandomReadHint(vma))
+ return;
+ if (ra->mmap_miss > 0)
+ ra->mmap_miss--;
+ if (PageReadahead(page))
+ page_cache_async_readahead(mapping, ra, file, page, offset, 1);
+}
+
/**
* filemap_fault - read in file data for page fault handling
* @vma: vma in which the fault was taken
@@ -1474,78 +1536,44 @@ int filemap_fault(struct vm_area_struct
struct address_space *mapping = file->f_mapping;
struct file_ra_state *ra = &file->f_ra;
struct inode *inode = mapping->host;
+ pgoff_t offset = vmf->pgoff;
struct page *page;
pgoff_t size;
- int did_readaround = 0;
int ret = 0;

size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
- if (vmf->pgoff >= size)
+ if (offset >= size)
return VM_FAULT_SIGBUS;

- /* If we don't want any read-ahead, don't bother */
- if (VM_RandomReadHint(vma))
- goto no_cached_page;
-
/*
* Do we have something in the page cache already?
*/
-retry_find:
- page = find_lock_page(mapping, vmf->pgoff);
- /*
- * For sequential accesses, we use the generic readahead logic.
- */
- if (VM_SequentialReadHint(vma)) {
- if (!page) {
- page_cache_sync_readahead(mapping, ra, file,
- vmf->pgoff, 1);
- page = find_lock_page(mapping, vmf->pgoff);
- if (!page)
- goto no_cached_page;
- }
- if (PageReadahead(page)) {
- page_cache_async_readahead(mapping, ra, file, page,
- vmf->pgoff, 1);
- }
- }
-
- if (!page) {
- unsigned long ra_pages;
-
- ra->mmap_miss++;
-
+ page = find_get_page(mapping, offset);
+ if (likely(page)) {
/*
- * Do we miss much more than hit in this file? If so,
- * stop bothering with read-ahead. It will only hurt.
+ * We found the page, so try async readahead before
+ * waiting for the lock.
*/
- if (ra->mmap_miss > MMAP_LOTSAMISS)
- goto no_cached_page;
+ do_async_mmap_readahead(vma, ra, file, page, offset);
+ lock_page(page);

- /*
- * To keep the pgmajfault counter straight, we need to
- * check did_readaround, as this is an inner loop.
- */
- if (!did_readaround) {
- ret = VM_FAULT_MAJOR;
- count_vm_event(PGMAJFAULT);
- }
- did_readaround = 1;
- ra_pages = max_sane_readahead(file->f_ra.ra_pages);
- if (ra_pages) {
- pgoff_t start = 0;
-
- if (vmf->pgoff > ra_pages / 2)
- start = vmf->pgoff - ra_pages / 2;
- do_page_cache_readahead(mapping, file, start, ra_pages);
+ /* Did it get truncated? */
+ if (unlikely(page->mapping != mapping)) {
+ unlock_page(page);
+ put_page(page);
+ goto no_cached_page;
}
- page = find_lock_page(mapping, vmf->pgoff);
+ } else {
+ /* No page in the page cache at all */
+ do_sync_mmap_readahead(vma, ra, file, offset);
+ count_vm_event(PGMAJFAULT);
+ ret = VM_FAULT_MAJOR;
+retry_find:
+ page = find_lock_page(mapping, offset);
if (!page)
goto no_cached_page;
}

- if (!did_readaround)
- ra->mmap_miss--;
-
/*
* We have a locked page in the page cache, now we need to check
* that it's up-to-date. If not, it is going to be due to an error.
@@ -1553,18 +1581,18 @@ retry_find:
if (unlikely(!PageUptodate(page)))
goto page_not_uptodate;

- /* Must recheck i_size under page lock */
+ /*
+ * Found the page and have a reference on it.
+ * We must recheck i_size under page lock.
+ */
size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
- if (unlikely(vmf->pgoff >= size)) {
+ if (unlikely(offset >= size)) {
unlock_page(page);
page_cache_release(page);
return VM_FAULT_SIGBUS;
}

- /*
- * Found the page and have a reference on it.
- */
- ra->prev_pos = (loff_t)page->index << PAGE_CACHE_SHIFT;
+ ra->prev_pos = (loff_t)offset << PAGE_CACHE_SHIFT;
vmf->page = page;
return ret | VM_FAULT_LOCKED;

@@ -1573,7 +1601,7 @@ no_cached_page:
* We're only likely to ever get here if MADV_RANDOM is in
* effect.
*/
- error = page_cache_read(file, vmf->pgoff);
+ error = page_cache_read(file, offset);

/*
* The page we want has now been added to the page cache.
@@ -1593,12 +1621,6 @@ no_cached_page:
return VM_FAULT_SIGBUS;

page_not_uptodate:
- /* IO error path */
- if (!did_readaround) {
- ret = VM_FAULT_MAJOR;
- count_vm_event(PGMAJFAULT);
- }
-
/*
* Umm, take care of errors if the page isn't up-to-date.
* Try to re-read it _once_. We do this synchronously,
--



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