On Sun, 21 Oct 2007 23:19:41 +0300 Artem Bityutskiy <[EMAIL PROTECTED]> wrote:
> Hi Andrew, > > some time ago we were talking about doing write-back from inside a > file-system > (http://marc.info/?l=linux-kernel&m=119097117713616&w=2). You said that I'm > not > the only person who needs this, because the same thing is needed for delayed > allocation. > > The problem is that if we initiate write-back from prepare_write() and we are > having a dirty page lock, we deadlock in write_cache_pages() which tries to > lock the same page. > > You suggested to enhance struct writeback_control and put page that should be > skipped. > > ... > > but it does not dot actually work, because if we have two processes forcing > write-back from write_page(), they will mutually deadlock (A waits in > write_cache_pages() on a page B has locked, B waits on inode or page A has > locked). Yeah, I was just thinking that as I read this ;) > So this way is not ok, do you have any other ideas? > > We could mark page clean temporarily before doing write-back, and mark it > dirty > again, but this seems to be inefficient (although I'm not sure, need to dig > these functions deeper, but they _seem_ to traverse the radix tree and change > tags, so marking one page dirty may need to change many tags, but again, I > did > not really dig tis yet). > > I'd appreciate any suggestions. Thanks! We could just skip locked pages altogether in writeback. Perhaps in WB_SYNC_NONE mode, or perhaps add a new flag in writeback_control to select this behaviour. It _should_ be the case that the number of locked-and-dirty pages which writeback encounters is very small, so skipping locked pages during writeback-for-memory-flushing won't have any significant effect. The first step should be to add a new /proc/vmstat field to count these pages and then do broad testing (especially on blocksize<pagesize filesystems) to confirm the theory. We'll still need to synchronously lock the page in writeback-for-data-integrity mode though. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
