Re: [PATCH] drm/xe: fix devcoredump chunk alignmnent calculation

[Matthew Brost <matthew.brost@intel.com>]

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 <arnd@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 <arnd@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
> >