[PATCH] mm: readahead: remove redundant ra_pages in file_ra_state

[Ying Zhu]

Hi,
  Recently we ran into the bug that an opened file's ra_pages does not
synchronize with it's backing device's when the latter is changed
with blockdev --setra, the application needs to reopen the file
to know the change, which is inappropriate under our circumstances.
This bug is also mentioned in scst (generic SCSI target subsystem for Linux)'s
README file.
  This patch tries to unify the ra_pages in struct file_ra_state 
and struct backing_dev_info. Basically current readahead algorithm 
will ramp file_ra_state.ra_pages up to bdi.ra_pages once it detects the 
read mode is sequential. Then all files sharing the same backing device 
have the same max value bdi.ra_pages set in file_ra_state. 
  Applying this means the flags POSIX_FADV_NORMAL and POSIX_FADV_SEQUENTIAL
in fadivse will only set file reading mode without signifying the
max readahead size of the file. The current apporach adds no additional
overhead in read IO path, IMHO is the simplest solution. 
Any comments are welcome, thanks in advance.

Thanks,
	Ying Zhu

Signed-off-by: Ying Zhu <casualfisher@xxxxxxxxx>
---
 include/linux/fs.h |    1 -
 mm/fadvise.c       |    2 --
 mm/filemap.c       |   17 +++++++++++------
 mm/readahead.c     |    8 ++++----
 4 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 17fd887..36303a5 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -991,7 +991,6 @@ struct file_ra_state {
 	unsigned int async_size;	/* do asynchronous readahead when
 					   there are only # of pages ahead */
 
-	unsigned int ra_pages;		/* Maximum readahead window */
 	unsigned int mmap_miss;		/* Cache miss stat for mmap accesses */
 	loff_t prev_pos;		/* Cache last read() position */
 };
diff --git a/mm/fadvise.c b/mm/fadvise.c
index 469491e..75e2378 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -76,7 +76,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
 
 	switch (advice) {
 	case POSIX_FADV_NORMAL:
-		file->f_ra.ra_pages = bdi->ra_pages;
 		spin_lock(&file->f_lock);
 		file->f_mode &= ~FMODE_RANDOM;
 		spin_unlock(&file->f_lock);
@@ -87,7 +86,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
 		spin_unlock(&file->f_lock);
 		break;
 	case POSIX_FADV_SEQUENTIAL:
-		file->f_ra.ra_pages = bdi->ra_pages * 2;
 		spin_lock(&file->f_lock);
 		file->f_mode &= ~FMODE_RANDOM;
 		spin_unlock(&file->f_lock);
diff --git a/mm/filemap.c b/mm/filemap.c
index a4a5260..e7e4409 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1058,11 +1058,15 @@ EXPORT_SYMBOL(grab_cache_page_nowait);
  * readahead(R+4...B+3) => bang => read(R+4) => read(R+5) => ......
  *
  * It is going insane. Fix it by quickly scaling down the readahead size.
+ * It's hard to estimate how the bad sectors lay out, so to be conservative,
+ * set the read mode in random.
  */
 static void shrink_readahead_size_eio(struct file *filp,
 					struct file_ra_state *ra)
 {
-	ra->ra_pages /= 4;
+	spin_lock(&filp->f_lock);
+	filp->f_mode |= FMODE_RANDOM;
+	spin_unlock(&filp->f_lock);
 }
 
 /**
@@ -1527,12 +1531,12 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
 	/* If we don't want any read-ahead, don't bother */
 	if (VM_RandomReadHint(vma))
 		return;
-	if (!ra->ra_pages)
+	if (!mapping->backing_dev_info->ra_pages)
 		return;
 
 	if (VM_SequentialReadHint(vma)) {
-		page_cache_sync_readahead(mapping, ra, file, offset,
-					  ra->ra_pages);
+		page_cache_sync_readahead(mapping, ra, file, offset, 
+					mapping->backing_dev_info->ra_pages);
 		return;
 	}
 
@@ -1550,7 +1554,7 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
 	/*
 	 * mmap read-around
 	 */
-	ra_pages = max_sane_readahead(ra->ra_pages);
+	ra_pages = max_sane_readahead(mapping->backing_dev_info->ra_pages);
 	ra->start = max_t(long, 0, offset - ra_pages / 2);
 	ra->size = ra_pages;
 	ra->async_size = ra_pages / 4;
@@ -1576,7 +1580,8 @@ static void do_async_mmap_readahead(struct vm_area_struct *vma,
 		ra->mmap_miss--;
 	if (PageReadahead(page))
 		page_cache_async_readahead(mapping, ra, file,
-					   page, offset, ra->ra_pages);
+					   page, offset, 
+					   mapping->backing_dev_info->ra_pages);
 }
 
 /**
diff --git a/mm/readahead.c b/mm/readahead.c
index ea8f8fa..6ea5999 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -27,7 +27,6 @@
 void
 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping)
 {
-	ra->ra_pages = mapping->backing_dev_info->ra_pages;
 	ra->prev_pos = -1;
 }
 EXPORT_SYMBOL_GPL(file_ra_state_init);
@@ -400,7 +399,8 @@ ondemand_readahead(struct address_space *mapping,
 		   bool hit_readahead_marker, pgoff_t offset,
 		   unsigned long req_size)
 {
-	unsigned long max = max_sane_readahead(ra->ra_pages);
+	struct backing_dev_info *bdi = mapping->backing_dev_info;
+	unsigned long max = max_sane_readahead(bdi->ra_pages);
 
 	/*
 	 * start of file
@@ -507,7 +507,7 @@ void page_cache_sync_readahead(struct address_space *mapping,
 			       pgoff_t offset, unsigned long req_size)
 {
 	/* no read-ahead */
-	if (!ra->ra_pages)
+	if (!mapping->backing_dev_info->ra_pages)
 		return;
 
 	/* be dumb */
@@ -543,7 +543,7 @@ page_cache_async_readahead(struct address_space *mapping,
 			   unsigned long req_size)
 {
 	/* no read-ahead */
-	if (!ra->ra_pages)
+	if (!mapping->backing_dev_info->ra_pages)
 		return;
 
 	/*
-- 
1.7.1

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