On Sun, Feb 04, 2007 at 02:30:55AM -0800, Andrew Morton wrote:
> On Sun, 4 Feb 2007 11:15:29 +0100 Nick Piggin <[email protected]> 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.
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