From: Usama Arif <usama.arif@bytedance.com>
To: dan.j.williams@intel.com, vishal.l.verma@intel.com,
dave.jiang@intel.com, nvdimm@lists.linux.dev
Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
Fam Zheng <fam.zheng@bytedance.com>,
"liangma@liangbit.com" <liangma@liangbit.com>
Subject: Conditions for FOLL_LONGTERM mapping in fsdax
Date: Mon, 6 Nov 2023 18:15:39 +0000 [thread overview]
Message-ID: <454dbfa1-2120-1e40-2582-d661203decca@bytedance.com> (raw)
In-Reply-To: <172ab047-0dc7-1704-5f30-ec7cd3632e09@bytedance.com>
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@dwillia2-desk3.amr.corp.intel.com/
Regards,
Usama
next parent reply other threads:[~2023-11-06 18:15 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <172ab047-0dc7-1704-5f30-ec7cd3632e09@bytedance.com>
2023-11-06 18:15 ` Usama Arif [this message]
2023-11-20 15:49 ` Conditions for FOLL_LONGTERM mapping in fsdax Usama Arif
2023-11-21 4:46 ` Christoph Hellwig
2023-11-27 11:52 ` [External] " Usama Arif
2023-11-27 14:00 ` Christoph Hellwig
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