[PATCH] remove throttle_vm_writeout()

Miklos Szeredi
2007-10-04T12:26:23Z

This in preparation for the writable mmap patches for fuse. I know it
conflicts with

writeback-remove-unnecessary-wait-in-throttle_vm_writeout.patch

but if this function is to be removed, it doesn't make much sense to
fix it first ;)
---

From: Miklos Szeredi <mszeredi@suse.cz>

By relying on the global diry limits, this can cause a deadlock when
devices are stacked.

If the stacking is done through a fuse filesystem, the __GFP_FS,
__GFP_IO tests won't help: the process doing the allocation doesn't
have any special flag.

So why exactly does this function exist?

Direct reclaim does not _increase_ the number of dirty pages in the
system, so rate limiting it seems somewhat pointless.

There are two cases:

1) File backed pages -> file

dirty + writeback count remains constant

2) Anonymous pages -> swap

writeback count increases, dirty balancing will hold back file
writeback in favor of swap

So the real question is: does case 2 need rate limiting, or is it OK
to let the device queue fill with swap pages as fast as possible?

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
---

Index: linux/include/linux/writeback.h
===================================================================
--- linux.orig/include/linux/writeback.h 2007-10-02 16:55:03.000000000 +0200
+++ linux/include/linux/writeback.h 2007-10-04 13:40:33.000000000 +0200
@@ -94,7 +94,6 @@ static inline void inode_sync_wait(struc
int wakeup_pdflush(long nr_pages);
void laptop_io_completion(void);
void laptop_sync_completion(void);
-void throttle_vm_writeout(gfp_t gfp_mask);

/* These are exported to sysctl. */
extern int dirty_background_ratio;
Index: linux/mm/page-writeback.c
===================================================================
--- linux.orig/mm/page-writeback.c 2007-10-02 16:55:03.000000000 +0200
+++ linux/mm/page-writeback.c 2007-10-04 13:40:33.000000000 +0200
@@ -497,37 +497,6 @@ void balance_dirty_pages_ratelimited_nr(
}
EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);

-void throttle_vm_writeout(gfp_t gfp_mask)
-{
- long background_thresh;
- long dirty_thresh;
-
- if ((gfp_mask & (__GFP_FS|__GFP_IO)) != (__GFP_FS|__GFP_IO)) {
- /*
- * The caller might hold locks which can prevent IO completion
- * or progress in the filesystem. So we cannot just sit here
- * waiting for IO to complete.
- */
- congestion_wait(WRITE, HZ/10);
- return;
- }
-
- for ( ; ; ) {
- get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
-
- /*
- * Boost the allowable dirty threshold a bit for page
- * allocators so they don't get DoS'ed by heavy writers
- */
- dirty_thresh += dirty_thresh / 10; /* wheeee... */
-
- if (global_page_state(NR_UNSTABLE_NFS) +
- global_page_state(NR_WRITEBACK) <= dirty_thresh)
- break;
- congestion_wait(WRITE, HZ/10);
- }
-}
-
/*
* writeback at least _min_pages, and keep writing until the amount of dirty
* memory is less than the background threshold, or until we're all clean.
Index: linux/mm/vmscan.c
===================================================================
--- linux.orig/mm/vmscan.c 2007-10-02 16:55:03.000000000 +0200
+++ linux/mm/vmscan.c 2007-10-04 13:40:33.000000000 +0200
@@ -1184,7 +1184,6 @@ static unsigned long shrink_zone(int pri
}
}

- throttle_vm_writeout(sc->gfp_mask);
return nr_reclaimed;
}