On Tue, Apr 29, 2025 at 09:34:00AM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <a...@arndb.de>
>
> The device core dumps are copied in 1.5GB chunks, which leads to a
> link-time error on 32-bit builds because of the 64-bit division not
> getting trivially turned into mask and shift operations:
>
> ERROR: modpost: "__moddi3" [drivers/gpu/drm/xe/xe.ko] undefined!
>
> On top of this, I noticed that the ALIGN_DOWN() usage here cannot
> work because that is only defined for power-of-two alignments.
> Change ALIGN_DOWN into an explicit div_u64_rem() that avoids the
> link error and hopefully produces the right results.
>
> Doing a 1.5GB kvmalloc() does seem a bit suspicious as well, e.g.
> this will clearly fail on any 32-bit platform and is also likely
> to run out of memory on 64-bit systems under memory pressure, so
> using a much smaller power-of-two chunk size might be a good idea
> instead.
>
> Fixes: c4a2e5f865b7 ("drm/xe: Add devcoredump chunking")
> Signed-off-by: Arnd Bergmann <a...@arndb.de>
Thanks for the fix, I had similar one [1] but I missed issue with
ALIGN_DOWN.
[1] https://patchwork.freedesktop.org/series/148301/
> ---
> Please test this with multi-gigabyte buffers, the original code
> was clearly not right, but I don't trust my version either.
This was tested on 64-bit only. I do see an issue with this version
though. Inline below.
> ---
> drivers/gpu/drm/xe/xe_devcoredump.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/xe/xe_devcoredump.c
> b/drivers/gpu/drm/xe/xe_devcoredump.c
> index a9e618abf8ac..4eb70e2d9f68 100644
> --- a/drivers/gpu/drm/xe/xe_devcoredump.c
> +++ b/drivers/gpu/drm/xe/xe_devcoredump.c
> @@ -177,6 +177,7 @@ static ssize_t xe_devcoredump_read(char *buffer, loff_t
> offset,
> struct xe_devcoredump *coredump = data;
> struct xe_devcoredump_snapshot *ss;
> ssize_t byte_copied;
> + u32 chunk_offset;
>
> if (!coredump)
> return -ENODEV;
> @@ -203,8 +204,9 @@ static ssize_t xe_devcoredump_read(char *buffer, loff_t
> offset,
>
> if (offset >= ss->read.chunk_position + XE_DEVCOREDUMP_CHUNK_MAX ||
> offset < ss->read.chunk_position) {
> - ss->read.chunk_position =
> - ALIGN_DOWN(offset, XE_DEVCOREDUMP_CHUNK_MAX);
> + ss->read.chunk_position = div_u64_rem(offset,
> + XE_DEVCOREDUMP_CHUNK_MAX, &chunk_offset)
> + * XE_DEVCOREDUMP_CHUNK_MAX;
>
> __xe_devcoredump_read(ss->read.buffer,
> XE_DEVCOREDUMP_CHUNK_MAX,
> @@ -213,8 +215,7 @@ static ssize_t xe_devcoredump_read(char *buffer, loff_t
> offset,
>
> byte_copied = count < ss->read.size - offset ? count :
> ss->read.size - offset;
> - memcpy(buffer, ss->read.buffer +
> - (offset % XE_DEVCOREDUMP_CHUNK_MAX), byte_copied);
> + memcpy(buffer, ss->read.buffer + chunk_offset, byte_copied);
chunk_offset is unset unless a new devcoredump is read which is every
1.5 GB. You will need always call div_u64_rem outside of the above if
statement.
Matt
>
> mutex_unlock(&coredump->lock);
>
> --
> 2.39.5
>
