Hi,
We wanted to run a VM with a vfio device assigned to it and with its
memory-backend-file residing in a persistent memory using fsdax (mounted
as ext4). It doesnt currently work with the kernel as
vfio_pin_pages_remote ends up requesting pages with FOLL_LONGTERM which
is currently not supported. From reading the mailing list, what I
understood was that this is to do with not having DMA supported on fsdax
due to issues that come up during truncate/hole-punching. But it was
solved with [1] by deferring fallocate(), truncate() on a dax mode file
while any page/block in the file is under active DMA.
If I remove the check which fails the gup opertion with the below diff,
the VM boots and the vfio device works without any issues. If I try to
truncate the mem file in fsdax, I can see that the truncate command gets
deferred (waits in ext4_break_layouts) and the vfio device keeps working
and sending packets without any issues. Just wanted to check what is
missing to allow FOLL_LONGTERM gup operations with fsdax? Is it just
enough to remove the check? Thanks!
diff --git a/mm/gup.c b/mm/gup.c
index eb8d7baf9e4d..f77bb428cf9b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1055,9 +1055,6 @@ static int check_vma_flags(struct vm_area_struct
*vma, unsigned long gup_flags)
if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma))
return -EFAULT;
- if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma))
- return -EOPNOTSUPP;
-
if (vma_is_secretmem(vma))
return -EFAULT;
[1]
https://lore.kernel.org/all/152669371377.34337.10697370528066177062.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Regards,
Usama
