Messages in this thread | | | Date | Mon, 31 Dec 2001 11:37:20 -0800 | From | Andrew Morton <> | Subject | Re: locked page handling |
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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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