Re: [PATCH v8 00/17] gfs2: Fix mmap + page fault deadlocks

[Catalin Marinas]

On Fri, Oct 29, 2021 at 12:15:55AM +0200, Andreas Grünbacher wrote:
> Am Do., 28. Okt. 2021 um 23:21 Uhr schrieb Catalin Marinas
> <catalin.marinas@xxxxxxx>:
> > I think for nested contexts we can save the uaccess fault state on
> > exception entry, restore it on return. Or (needs some thinking on
> > atomicity) save it in a local variable. The high-level API would look
> > something like:
> >
> >         unsigned long uaccess_flags;    /* we could use TIF_ flags */
> >
> >         uaccess_flags = begin_retriable_uaccess();
> >         copied = copy_page_from_iter_atomic(...);
> >         retry = end_retriable_uaccess(uaccess_flags);
> >         ...
> >
> >         if (!retry)
> >                 break;
> >
> > I think we'd need a TIF flag to mark the retriable region and another to
> > track whether a non-recoverable fault occurred. It needs prototyping.
> >
> > Anyway, if you don't like this approach, I'll look at error codes being
> > returned but rather than changing all copy_from_user() etc., introduce a
> > new API that returns different error codes depending on the fault
> > (e.g -EFAULT vs -EACCES). We already have copy_from_user_nofault(), we'd
> > need something for the iov_iter stuff to use in the fs code.
> 
> We won't need any of that on the filesystem read and write paths. The
> two cases there are buffered and direct I/O:

Thanks for the clarification, very useful.

> * In the buffered I/O case, the copying happens with page faults
> disabled, at a byte granularity. If that returns a short result, we
> need to enable page faults, check if the exact address that failed
> still fails (in which case we have a sub-page fault),  fault in the
> pages, disable page faults again, and repeat. No probing for sub-page
> faults beyond the first byte of the fault-in address is needed.
> Functions fault_in_{readable,writeable} implicitly have this behavior;
> for fault_in_safe_writeable() the choice we have is to either add
> probing of the first byte for sub-page faults to this function or
> force callers to do that probing separately. At this point, I'd vote
> for the former.

This sounds fine to me (and I have some draft patches already on top of
your series).

> * In the direct I/O case, the copying happens while we're holding page
> references, so the only page faults that can occur during copying are
> sub-page faults.

Does holding a page reference guarantee that the user pte pointing to
such page won't change, for example a pte_mkold()? I assume for direct
I/O, the PG_locked is not held. But see below, it may not be relevant.

> When iomap_dio_rw or its legacy counterpart is called
> with page faults disabled, we need to make sure that the caller can
> distinguish between page faults triggered during
> bio_iov_iter_get_pages() and during the copying, but that's a separate
> problem. (At the moment, when iomap_dio_rw fails with -EFAULT, the
> caller *cannot* distinguish between a bio_iov_iter_get_pages failure
> and a failure during synchronous copying, but that could be fixed by
> returning unique error codes from iomap_dio_rw.)

Since the direct I/O pins the pages in memory, does it even need to do a
uaccess? It could copy the data via the kernel mapping (kmap). For arm64
MTE, all such accesses are not checked (they use a match-all pointer
tag) since the kernel is not set up to handle such sub-page faults (no
copy_from/to_user but a direct access).

> So as far as I can see, the only problematic case we're left with is
> copying bigger than byte-size chunks with page faults disabled when we
> don't know whether the underlying pages are resident or not. My guess
> would be that in this case, if the copying fails, it would be
> perfectly acceptable to explicitly probe the entire chunk for sub-page
> faults.

Yeah, if there are only a couple of places left, we can add the explicit
probing (via some probe_user_writable function).

-- 
Catalin