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Subjectlocked page handling


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 ?
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

-- Amol





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