>> > > And for the vmscan->writepage() side of things I wonder if it would be >> > > possible to overload the mapping's ->nopage handler. If the target page >> > > lies in a hole, go off and allocate all the necessary pagecache pages, zero >> > > them, mark them dirty? >> > >> > I guess it would be possible but ->nopage is used for the read case and >> > why would we want to then cause writes/allocations? >> >> yup, we'd need to create a new handler for writes, or pass `write_access' >> into ->nopage. I think others (dwdm2?) have seen a need for that. > >That would work as long as all writable mappings are actually written to >everywhere. Otherwise you still get that reading the whole mmap()ped >are but writing a small part of it would still instantiate all of it on >disk. As far as I understand this there is no way to hook into the mmap >system such that we have a hook whenever a mmap()ped page gets written >to for the first time. (I may well be wrong on that one so please >correct me if that is the case.)
I think the point is that we can't have a "handler for writes," because the writes are being done by simple CPU Store instructions in a user program. The handler we're talking about is just for page faults. Other operating systems approach this by actually _having_ a handler for a CPU store instruction, in the form of a page protection fault handler -- the nopage routine adds the page to the user's address space, but write protects it. The first time the user tries to store into it, the filesystem driver gets a chance to do what's necessary to support a dirty cache page -- allocate a block, add additional dirty pages to the cache, etc. It would be wonderful to have that in Linux. I saw hints of such code in a Linux kernel once (a "write_protect" address space operation or something like that); I don't know what happened to it. Short of that, I don't see any way to avoid sometimes filling in holes due to reads. It's not a huge problem, though -- it requires someone to do a shared writable mmap and then read lots of holes and not write to them, which is a pretty rare situation for a normal file. I didn't follow how the helper function solves this problem. If it's something involving adding the required extra pages to the cache at pageout time, then that's not going to work -- you can't make adding pages to the cache a prerequisite for cleaning a page -- that would be Deadlock City. My large-block filesystem driver does the nopage thing, and does in fact fill in files unnecessarily in this scenario. :-( The driver for the same filesystems on AIX does not, though. It has the write protection thing. -- Bryan Henderson IBM Almaden Research Center San Jose CA Filesystems - 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/
