Re: [VM PATCH 2.6.6-rc3-bk5] Dirty balancing in the presence of mapped pages

[Andrew Morton <akpm@osdl.org>]

Shantanu Goel <sgoel01@yahoo.com> wrote:
>
> > In this case, given that we have an actively mapped
>  > MAP_SHARED pagecache
>  > page, marking it dirty will cause it to be written
>  > by pdflush.  Even though
>  > we're not about to reclaim it, and even though the
>  > process which is mapping
>  > the page may well modify it again.  This patch will
>  > cause additional I/O.
>  > 
> 
>  True, but is that really very different from normal
>  file I/O where we actively balance # dirty pages? 
>  Also, the I/O will only happen if the dirty thresholds
>  are exceeded.  It probably makes sense though to skip
>  SwapCache pages to more closely mimic file I/O
>  behaviour.

We need to think about why real applications (as opposed to benchmarks) use
MAP_SHARED.  I suspect many of them will modify pages again and again and
again.  We really want to avoid writing these pages out until the
application has truly finished with them.

I think it is probably the case that pages which were dirtied with write(2)
are much less likely to be redirtied than pages which were dirtied via
MAP_SHARED.

One thing you might like to look at is to give these pages another trip
around the LRU after they have been unmapped from pagetables, and to give
pdflush a poke.  Add instrumentation to record how many pages end up
getting written via vmscan's writepage versus via pdflush (use
current_is_pdflush()).


diff -puN mm/vmscan.c~a mm/vmscan.c
--- 25/mm/vmscan.c~a	2004-05-04 22:21:41.613856240 -0700
+++ 25-akpm/mm/vmscan.c	2004-05-04 22:23:16.016504856 -0700
@@ -318,7 +318,9 @@ shrink_list(struct list_head *page_list,
 				rmap_unlock(page);
 				goto keep_locked;
 			case SWAP_SUCCESS:
-				; /* try to free the page below */
+				if (PageDirty(page))
+					goto keep_locked;
+				break;
 			}
 		}
 		rmap_unlock(page);

_