On Sun, Feb 04, 2007 at 02:56:02AM -0800, Andrew Morton wrote: > On Sun, 4 Feb 2007 11:46:09 +0100 Nick Piggin <npiggin@xxxxxxx> wrote: > > > On Sun, Feb 04, 2007 at 02:30:55AM -0800, Andrew Morton wrote: > > > On Sun, 4 Feb 2007 11:15:29 +0100 Nick Piggin <npiggin@xxxxxxx> wrote: > > > > > > > The write path is broken. I prefer my kernels slow, than buggy. > > > > > > That won't fly. > > > > What won't fly? > > I suspect the performance cost of this approach would force us to redo it > all. That's the idea. But at least in the meantime we're correct. > > > > That was my second idea. > > > > > > Coulda sworn it was mine ;) I thought you ended up deciding it wasn't > > > practical because of the games we needed to play with ->commit_write. > > > > Maybe I misunderstood what you meant, above. > > The original set of half-written patches I sent you. Do an atomic copy_from_user() > inside the page lock and if that fails, zero out the remainder of the page, run > commit_write() and then redo the whole thing. Oh that. Data corruption, transient zeroes. > > I have an alterative fix > > where a temporary page is allocated if the write enncounters a non > > uptodate page. The usercopy then goes into that page, and from there > > into the target page after we have opened the prepare_write(). > > Remember that a non-uptodate page is the common case. Yes. > > > My *first* idea to fix this was to do the atomic copy into a non-uptodate > > page and then calling a zero-length commit_write if it failed. I pretty > > carefully constructed all these good arguments as to why each case works > > properly, but in the end it just didn't fly because it broke lots of > > filesystems. > > I forget the details now. I think we did have a workable-looking solution > based on the atomic copy_from_user() but it would have re-exposed the old > problem wherein a page would fleetingly have a bunch of zeroes in the > middle of it, if someone looked at it during the write. > > If that recollection is right, I think we could afford to reintroduce that > problem, frankly. Especially as it only happens in the incredibly rare > case of that get_user()ed page getting unmapped under our feet. Dang. I was hoping to fix it without introducing data corruption. > > > > but you introduce the theoretical memory deadlock > > > > where a task cannot reclaim its own memory. > > > > > > Nah, that'll never happen - both pages are already allocated. > > > > Both pages? I don't get it. > > > > You set the don't-reclaim vma flag, then run get_user, which takes a > > page fault and potentially has to allocate N pages for pagetables, > > pagecache readahead, buffers and fs private data and pagecache radix > > tree nodes for all of the pages read in. > > Oh, OK. Need to do the get_user() twice then. Once before taking that new > rwsem. Race condition remains. - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
