On Tue, Apr 29, 2025 at 10:51:18AM -0700, Matthew Brost wrote:
> 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.
>
Oh, yea wrt the ALIGN_DOWN we'd be reading the wrong data. We have WIP
to replay hangs on the GPU and this would break the tool. Good catch.
Matt
> > ---
> > 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
> >
