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writeback: pay attention to wbc->nr_to_write in write_cache_pages

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If a filesystem writes more than one page in ->writepage, write_cache_pages
fails to notice this and continues to attempt writeback when wbc->nr_to_write
has gone negative - this trace was captured from XFS:

    wbc_writeback_start: towrt=1024
    wbc_writepage: towrt=1024
    wbc_writepage: towrt=0
    wbc_writepage: towrt=-1
    wbc_writepage: towrt=-5
    wbc_writepage: towrt=-21
    wbc_writepage: towrt=-85

This has adverse effects on filesystem writeback behaviour. write_cache_pages()
needs to terminate after a certain number of pages are written, not after a
certain number of calls to ->writepage are made.  This is a regression
introduced by 17bc6c30cf ("vfs: Add
no_nrwrite_index_update writeback control flag"), but cannot be reverted
directly due to subsequent bug fixes that have gone in on top of it.

Signed-off-by: Dave Chinner <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Dave Chinner 2010-06-09 10:37:18 +10:00 • committed by Linus Torvalds
commit 0b5649278e
3 changed files with 6 additions and 23 deletions
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15
mm/page-writeback.c
View file

@ -835,7 +835,6 @@ int write_cache_pages(struct address_space *mapping,
pgoff_t done_index;
int cycled;
int range_whole = 0;
long nr_to_write = wbc->nr_to_write;
pagevec_init(&pvec, 0);
if (wbc->range_cyclic) {
@ -935,11 +934,10 @@ continue_unlock:
done = 1;
break;
}
}
}
if (nr_to_write > 0) {
nr_to_write--;
if (nr_to_write == 0 &&
if (wbc->nr_to_write > 0) {
if (--wbc->nr_to_write == 0 &&
wbc->sync_mode == WB_SYNC_NONE) {
/*
* We stop writing back only if we are
@ -970,11 +968,8 @@ continue_unlock:
end = writeback_index - 1;
goto retry;
}
if (!wbc->no_nrwrite_index_update) {
if (wbc->range_cyclic || (range_whole && nr_to_write > 0))
mapping->writeback_index = done_index;
wbc->nr_to_write = nr_to_write;
}
if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
mapping->writeback_index = done_index;
return ret;
}