On Sun, 4 Feb 2007 11:15:29 +0100 Nick Piggin <npiggin@xxxxxxx> wrote: > On Sun, Feb 04, 2007 at 01:44:45AM -0800, Andrew Morton wrote: > > On Sun, 4 Feb 2007 09:51:07 +0100 (CET) Nick Piggin <npiggin@xxxxxxx> wrote: > > > > > 2. If we find the destination page is non uptodate, unlock it (this could be > > > made slightly more optimal), then find and pin the source page with > > > get_user_pages. Relock the destination page and continue with the copy. > > > However, instead of a usercopy (which might take a fault), copy the data > > > via the kernel address space. > > > > argh. We just can't go adding all this gunk into the write() path. > > > > mmap_sem, a full pte-walk, taking of pte-page locks, etc. For every page. > > Even single-process write() will suffer, let along multithreaded stuff, > > where mmap_sem contention may be the bigger problem. > > The write path is broken. I prefer my kernels slow, than buggy. That won't fly. > > There's a build error in filemap_xip.c btw. ? > > > > We need to think different. > > > > 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. > I didn't get any feedback on that patchset > except to try this method, so I assume everyone hated it. > > I actually liked it, because it didn't have to do the writev > segment-at-a-time for !uptodate pages like this one does. Considering > this code gets called from mm-less contexts, maybe I'll have to go back > to this approach. OK. > > 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. > 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. It's better than taking mmap_sem and walking pagetables... - 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
