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? > > > > There's a build error in filemap_xip.c btw. > > ? Thanks? > > > What happened to the idea of doing an atomic copy into the non-uptodate > > > page and handling it somehow? > > > > 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. 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(). 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. > > > Another option might be to effectively pin the whole mm during the copy: > > > > > > down_read(¤t->mm->unpaging_lock); > > > get_user(addr); /* Fault the page in */ > > > ... > > > copy_from_user() > > > up_read(¤t->mm->unpaging_lock); > > > > > > then, anyone who wants to unmap pages from this mm requires > > > write_lock(unpaging_lock). So we know the results of that get_user() > > > cannot be undone. > > > > Fugly. > > I invited you to think different - don't just fixate on one random > tossed-out-there suggestion. I've thought. Quite a lot. I have 2 other approaches that don't require mmap_sem, and 1 which is actually possible to implement without breaking filesystems. > > 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. > It's better than taking mmap_sem and walking pagetables... I'm not convinced. - 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
