Re: Conditions for FOLL_LONGTERM mapping in fsdax

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On 06/11/2023 18:15, Usama Arif wrote:
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/


Hi,

Just wanted to check if there were any comments on this?

Thanks


Regards,
Usama




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