Messages in this thread | | | Date | Wed, 11 Mar 2009 23:36:15 +0100 | From | Johannes Weiner <> | Subject | Re: [PATCH] NOMMU: Pages allocated to a ramfs inode's pagecache may get wrongly discarded |
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On Wed, Mar 11, 2009 at 03:03:02PM -0700, Andrew Morton wrote: > On Wed, 11 Mar 2009 15:30:35 +0000 > David Howells <dhowells@redhat.com> wrote: > > > From: Enrik Berkhan <Enrik.Berkhan@ge.com> > > > > The pages attached to a ramfs inode's pagecache by truncation from nothing - as > > done by SYSV SHM for example - may get discarded under memory pressure. > > Something has gone wrong in core VM. > > > The problem is that the pages are not marked dirty. Anything that creates data > > in an MMU-based ramfs will cause the pages holding that data will cause the > > set_page_dirty() aop to be called. > > > > For the NOMMU-based mmap, set_page_dirty() may be called by write(), but it > > won't be called by page-writing faults on writable mmaps, and it isn't called > > by ramfs_nommu_expand_for_mapping() when a file is being truncated from nothing > > to allocate a contiguous run. > > > > The solution is to mark the pages dirty at the point of allocation by > > the truncation code. > > Page reclaim shouldn't be even attempting to reclaim or write back > ramfs pagecache pages - reclaim can't possibly do anything with these > pages! > > Arguably those pages shouldn't be on the LRU at all, but we haven't > done that yet. > > Now, my problem is that I can't 100% be sure that we _ever_ implemented > this properly. I _think_ we did, in which case we later broke it. If > we've always been (stupidly) trying to pageout these pages then OK, I > guess your patch is a suitable 2.6.29 stopgap. > > If, however, we broke it then we've probably broken other filesystems > and we should fix the regression instead. > > Running bdi_cap_writeback_dirty() in may_write_to_queue() might be the > way to fix all this.
The pages are not dirty, so no pageout() which says PAGE_KEEP. It will just go through and reclaim the clean, unmapped pages.
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