Andries.Brouwer@cwi.nl wrote:
>
> ...
> Do these cards not have a request queue?
>
> The kernel views them as SCSI disks.
> So yes, I can do
>
> blockdev --setra 0 /dev/sdc
>
> Unfortunately that does not help in the least.
> Indeed, the only user of the readahead info is
> readahead.c: get_max_readahead() and it does
>
> blk_ra_kbytes = blk_get_readahead(inode->i_dev) / 2;
> if (blk_ra_kbytes < VM_MIN_READAHEAD)
> blk_ra_kbytes = VM_MAX_READAHEAD;
>
> We need to distinguish between undefined, and explicily zero.
Yup. The below (untested) patch should fix it up. Assuming
that all drivers use blk_init_queue(), and heaven knows if
that's the case. If not, the readahead window will have to be
set from userspace for those devices.
> ...
>
> Yes, but things should be OK as-is. If the readahead attempt
> gets an I/O error, do_generic_file_read will notice the non-uptodate
> page and will issue a single-page read. So everything up to
> a page's distance from the bad block should be recoverable.
> That's the theory; can't say that I've tested it.
>
> It is really important to be able to tell the kernel to read and
> write only the blocks it has been asked to read and write and
> not to touch anything else.
That is really hard when there is a filesystem mounted.
Even with readahead set to zero, the kernel will go and
read PAGE_CACHE_SIZE chunks. That's not worth changing
to address this problem. In the last resort, the
user would need to perform a sector-by-sector read of
the dud device into a regular file, recover from that.
--- 2.5.8/mm/readahead.c~readahead-fixes Tue Apr 16 12:07:42 2002
+++ 2.5.8-akpm/mm/readahead.c Tue Apr 16 12:13:52 2002
@@ -25,9 +25,6 @@
* has a zero value of ra_sectors.
*/
-#define VM_MAX_READAHEAD 128 /* kbytes */
-#define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */
-
/*
* Return max readahead size for this inode in number-of-pages.
*/
@@ -36,9 +33,6 @@ static int get_max_readahead(struct inod
unsigned blk_ra_kbytes = 0;
blk_ra_kbytes = blk_get_readahead(inode->i_dev) / 2;
- if (blk_ra_kbytes < VM_MIN_READAHEAD)
- blk_ra_kbytes = VM_MAX_READAHEAD;
-
return blk_ra_kbytes >> (PAGE_CACHE_SHIFT - 10);
}
@@ -96,10 +90,10 @@ static int get_min_readahead(struct inod
* second page) then the window will build more slowly.
*
* On a readahead miss (the application seeked away) the readahead window is shrunk
- * by 25%. We don't want to drop it too aggressively, because it's a good assumption
- * that an application which has built a good readahead window will continue to
- * perform linear reads. Either at the new file position, or at the old one after
- * another seek.
+ * by 25%. We don't want to drop it too aggressively, because it is a good
+ * assumption that an application which has built a good readahead window will
+ * continue to perform linear reads. Either at the new file position, or at the
+ * old one after another seek.
*
* There is a special-case: if the first page which the application tries to read
* happens to be the first page of the file, it is assumed that a linear read is
@@ -109,9 +103,9 @@ static int get_min_readahead(struct inod
* sequential file reading.
*
* This function is to be called for every page which is read, rather than when
- * it is time to perform readahead. This is so the readahead algorithm can centrally
- * work out the access patterns. This could be costly with many tiny read()s, so
- * we specifically optimise for that case with prev_page.
+ * it is time to perform readahead. This is so the readahead algorithm can
+ * centrally work out the access patterns. This could be costly with many tiny
+ * read()s, so we specifically optimise for that case with prev_page.
*/
/*
@@ -208,8 +202,10 @@ void page_cache_readahead(struct file *f
goto out;
}
- min = get_min_readahead(inode);
max = get_max_readahead(inode);
+ if (max == 0)
+ goto out; /* No readahead */
+ min = get_min_readahead(inode);
if (ra->next_size == 0 && offset == 0) {
/*
@@ -310,7 +306,8 @@ void page_cache_readaround(struct file *
{
unsigned long target;
unsigned long backward;
- const int min = get_min_readahead(file->f_dentry->d_inode->i_mapping->host) * 2;
+ const int min =
+ get_min_readahead(file->f_dentry->d_inode->i_mapping->host) * 2;
if (file->f_ra.next_size < min)
file->f_ra.next_size = min;
--- 2.5.8/include/linux/mm.h~readahead-fixes Tue Apr 16 12:11:39 2002
+++ 2.5.8-akpm/include/linux/mm.h Tue Apr 16 12:12:14 2002
@@ -539,6 +539,8 @@ extern int filemap_sync(struct vm_area_s
extern struct page *filemap_nopage(struct vm_area_struct *, unsigned long, int);
/* readahead.c */
+#define VM_MAX_READAHEAD 128 /* kbytes */
+#define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */
void do_page_cache_readahead(struct file *file,
unsigned long offset, unsigned long nr_to_read);
void page_cache_readahead(struct file *file, unsigned long offset);
--- 2.5.8/drivers/block/ll_rw_blk.c~readahead-fixes Tue Apr 16 12:12:19 2002
+++ 2.5.8-akpm/drivers/block/ll_rw_blk.c Tue Apr 16 12:13:43 2002
@@ -851,7 +851,7 @@ int blk_init_queue(request_queue_t *q, r
q->plug_tq.data = q;
q->queue_flags = (1 << QUEUE_FLAG_CLUSTER);
q->queue_lock = lock;
- q->ra_sectors = 0; /* Use VM default */
+ q->ra_sectors = VM_MAX_READAHEAD << (PAGE_CACHE_SHIFT - 9);
blk_queue_segment_boundary(q, 0xffffffff);
-
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Tue Apr 23 2002 - 22:00:16 EST