On Fri, Jul 01, 2022 at 07:56:28AM +0200, Juergen Gross wrote: > On 30.06.22 18:54, Demi Marie Obenour wrote: > > On Thu, Jun 30, 2022 at 03:16:41PM +0200, Juergen Gross wrote: > > > On 30.06.22 13:34, Greg KH wrote: > > > > On Mon, Jun 27, 2022 at 02:10:02PM -0400, Demi Marie Obenour wrote: > > > > > commit dbe97cff7dd9f0f75c524afdd55ad46be3d15295 upstream > > > > > > > > > > unmap_grant_pages() currently waits for the pages to no longer be used. > > > > > In https://github.com/QubesOS/qubes-issues/issues/7481, this lead to a > > > > > deadlock against i915: i915 was waiting for gntdev's MMU notifier to > > > > > finish, while gntdev was waiting for i915 to free its pages. I also > > > > > believe this is responsible for various deadlocks I have experienced in > > > > > the past. > > > > > > > > > > Avoid these problems by making unmap_grant_pages async. This requires > > > > > making it return void, as any errors will not be available when the > > > > > function returns. Fortunately, the only use of the return value is a > > > > > WARN_ON(), which can be replaced by a WARN_ON when the error is > > > > > detected. Additionally, a failed call will not prevent further calls > > > > > from being made, but this is harmless. > > > > > > > > > > Because unmap_grant_pages is now async, the grant handle will be sent to > > > > > INVALID_GRANT_HANDLE too late to prevent multiple unmaps of the same > > > > > handle. Instead, a separate bool array is allocated for this purpose. > > > > > This wastes memory, but stuffing this information in padding bytes is > > > > > too fragile. Furthermore, it is necessary to grab a reference to the > > > > > map before making the asynchronous call, and release the reference when > > > > > the call returns. > > > > > > > > > > It is also necessary to guard against reentrancy in gntdev_map_put(), > > > > > and to handle the case where userspace tries to map a mapping whose > > > > > contents have not all been freed yet. > > > > > > > > > > Fixes: 745282256c75 ("xen/gntdev: safely unmap grants in case they are still in use") > > > > > Cc: stable@xxxxxxxxxxxxxxx > > > > > Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx> > > > > > Reviewed-by: Juergen Gross <jgross@xxxxxxxx> > > > > > Link: https://lore.kernel.org/r/20220622022726.2538-1-demi@xxxxxxxxxxxxxxxxxxxxxx > > > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx> > > > > > --- > > > > > drivers/xen/gntdev-common.h | 7 ++ > > > > > drivers/xen/gntdev.c | 142 +++++++++++++++++++++++++----------- > > > > > 2 files changed, 106 insertions(+), 43 deletions(-) > > > > > > > > All now queued up, thanks. > > > > > > Sorry, but I think at least the version for 5.10 is fishy, as it removes > > > the tests for successful allocations of add->map_ops and add->unmap_ops. > > > > That is definitely a bug; I will send another version (without your > > Reviewed-by). > > > > > I need to do a thorough review of the patches (the "Reviewed-by:" tag in > > > the patches is the one for the upstream patch). > > > > Yeah, that was my fault, sorry. > > > > > Greg, can you please wait for my explicit "okay" for the backports? > > > > Confirming that these patches do need review before they can be applied. > > Juergen, would you mind making sure that the upstream patch was also > > correct for 5.15 and 5.18? It applied cleanly, but that is no guarantee > > of correctness. > > Those two are fine. Thanks! I re-sent the 5.10 patch; hopefully the others are good. -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab
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