Andrew Morton wrote:
>
> So... the big "how it all works" comment needs an update..
Same patch, comment updated.
Currently page_cache_readahead() treats ra->size == 0 (first read)
and ra->size == -1 (ra_off was called) separately, but does exactly
the same in both cases.
With this patch we may assume that the reading starts in 'ra_off()'
state, so we don't need to consider the first read as a special case.
file_ra_state_init() sets
ra->prev_page = -1;
ra->size = 0;
When the page_cache_readahead() is called for the first time it sets
ra->size to nonzero value either via get_init_ra_size() or ra_off().
So ra->size == 0 implies that ra->prev_page == -1. I am ignoring the
case when readahead is disabled via ra->ra_pages == 0.
page_cache_readahead detects sub-page sized reads:
if (offset == ra->prev_page && req_size == 1 && ra->size != 0)
But if offset == ra->prev_page, then ra->size == 0 can happen only if
offset == -1, so there is no need to check ra->size here. If application
starts reading 16Tb file from the last page then readahead can't help.
First offset==0 read or first sequential detection:
if ((ra->size == 0 && offset == 0) || (ra->size == -1 && sequential)
could be changed to:
if ((ra->size == 0 && sequential) || (ra->size == -1 && sequential)
which means:
if (sequential && (ra->size == 0 || ra->size == -1))
Random case detection:
if (!sequential || (ra->size == 0))
But if sequential == 1, then ra->size can't be 0, this case is already handled
before.
Now we have:
if (offset == ra->prev_page && req_size == 1)
/* sub-page reads */
if (sequential && (ra->size == 0 || ra->size == -1))
/* first offset==0 read or first sequential */
if (!sequential)
/* random case */
Now ->size is checked only in one place, so ra_off() can set ra->size = 0,
and we can just test ->size against 0.
Signed-off-by: Oleg Nesterov <[EMAIL PROTECTED]>
--- 2.6.11/mm/readahead.c~ 2005-02-04 21:33:40.000000000 +0300
+++ 2.6.11/mm/readahead.c 2005-02-04 21:33:57.000000000 +0300
@@ -55,7 +55,7 @@ static inline void ra_off(struct file_ra
{
ra->start = 0;
ra->flags = 0;
- ra->size = -1;
+ ra->size = 0;
ra->ahead_start = 0;
ra->ahead_size = 0;
return;
@@ -452,7 +452,7 @@ page_cache_readahead(struct address_spac
* perturbing the readahead window expansion logic.
* If size is zero, there is no read ahead window so we need one
*/
- if (offset == ra->prev_page && req_size == 1 && ra->size != 0)
+ if (offset == ra->prev_page && req_size == 1)
goto out;
ra->prev_page = offset;
@@ -471,9 +471,7 @@ page_cache_readahead(struct address_spac
* at start of file, and grow the window fast. Or detect first
* sequential access
*/
- if ((ra->size == 0 && offset == 0) /* first io and start of file */
- || (ra->size == -1 && sequential)) {
- /* First sequential */
+ if (sequential && ra->size == 0) {
ra->size = get_init_ra_size(newsize, max);
ra->start = offset;
if (!blockable_page_cache_readahead(mapping, filp, offset,
@@ -499,7 +497,7 @@ page_cache_readahead(struct address_spac
* partial page reads and first access were handled above,
* so this must be the next page otherwise it is random
*/
- if (!sequential || (ra->size == 0)) {
+ if (!sequential) {
ra_off(ra);
blockable_page_cache_readahead(mapping, filp, offset,
newsize, ra, 1);
-
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