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SubjectRe: kernel BUG at mm/hugetlb.c:LINE!
On Wed, May 20, 2020 at 10:27:15AM -0700, Mike Kravetz wrote:

> I am fairly confident it is all about checking limits and alignment. The
> filesystem knows if it can/should align to base or huge page size. DAX has
> some interesting additional restrictions, and several 'traditional' filesystems
> check if they are 'on DAX'.


Okay, I haven't looked at DAX vs. overlay. I'm sure it's going to come up at
some point, if it hasn't already.

>
> In a previous e-mail, you suggested hugetlb_get_unmapped_area could do the
> length adjustment in hugetlb_get_unmapped_area (generic and arch specific).
> I agree, although there may be the need to add length overflow checks in
> these routines (after round up) as this is done in core code now. However,
> this can be done as a separate cleanup patch.
>
> In any case, we need to get the core mmap code to call filesystem specific
> get_unmapped_area if on a union/overlay. The patch I suggested does this
> by simply calling real_file to determine if there is a filesystem specific
> get_unmapped_area. The other approach would be to provide an overlayfs
> get_unmapped_area that calls the underlying filesystem get_unmapped_area.

That latter is what's done for all other stacked operations in overlayfs.

Untested patch below.

Thanks,
Miklos

---
fs/overlayfs/file.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -757,6 +757,17 @@ static loff_t ovl_remap_file_range(struc
remap_flags, op);
}

+static unsigned long ovl_get_unmapped_area(struct file *file,
+ unsigned long uaddr, unsigned long len,
+ unsigned long pgoff, unsigned long flags)
+{
+ struct file *realfile = file->private_data;
+
+ return (realfile->f_op->get_unmapped_area ?:
+ current->mm->get_unmapped_area)(realfile,
+ uaddr, len, pgoff, flags);
+}
+
const struct file_operations ovl_file_operations = {
.open = ovl_open,
.release = ovl_release,
@@ -774,6 +785,7 @@ const struct file_operations ovl_file_op

.copy_file_range = ovl_copy_file_range,
.remap_file_range = ovl_remap_file_range,
+ .get_unmapped_area = ovl_get_unmapped_area,
};

int __init ovl_aio_request_cache_init(void)
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