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SubjectRe: locked page handling
alad@hss.hns.com wrote:
>
> In 2.4.16, vmscan.c::shrink_cache(), we have following piece of code -
>
> /*
> * The page is locked. IO in progress?
> * Move it to the back of the list.
> */
> if (unlikely(TryLockPage(page))) {
> if (PageLaunder(page) && (gfp_mask & __GFP_FS)) {
> page_cache_get(page);
> spin_unlock(&pagemap_lru_lock);
> wait_on_page(page);
> page_cache_release(page);
> spin_lock(&pagemap_lru_lock);
> }
> continue;
> }
>
> 1) Who is moving the page the back of list ?

Nobody. The comment is wrong.

Possibly the code is wrong, too. We don't want to keep scanning
the same page all the time.

> 2) Is the locked page worth waiting for? I can understand that the page is being
> laundered so after wait we may get a clean page but from performance
> point of view this is involving unnecessary context switches. Also during
> high memory pressure kswapd shall sleep here when it can get more
> clean pages on the inactive list ? What are we loosing if we don't wait on
> the page and believe that in next pass we shall free this page
>

Well we need to wait on I/O _somewhere_ in there. Otherwise everyone
just ends up busywaiting on IO completion. The idea is that on the
first pass through the inactive list, we start I/O, mark the page as
PG_Launder and don't wait on the I/O. On the second pass through the
list, when we find a PG_Launder page, we wait on it. This has the
effect of slowing memory-requesters down to the speed of the I/O
system. All this is for mmapped pages. The same behaviour is
implemented for write() pages via the BH_Launder bits on its buffers
over in sync_page_buffers().

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