Re: Excessive page cache occupies DMA32 memory

[Robin Murphy]

On 2025-07-22 8:24 am, Greg KH wrote:
> On Tue, Jul 22, 2025 at 11:05:11AM +0500, Muhammad Usama Anjum wrote:
> > Adding ath/mhi and dma API developers to the discussion.
> >
> > On 7/22/25 10:32 AM, Greg KH wrote:
> > > On Mon, Jul 21, 2025 at 06:13:10PM +0100, Matthew Wilcox wrote:
> > > > On Mon, Jul 21, 2025 at 08:03:12PM +0500, Muhammad Usama Anjum wrote:
> > > > > Hello,
> > > > >
> > > > > When 10-12GB our of total 16GB RAM is being used as page cache
> > > > > (active_file + inactive_file) at suspend time, the drivers fail to allocate
> > > > > dma memory at resume as dma memory is either occupied by the page cache or
> > > > > fragmented. Example:
> > > > >
> > > > > kworker/u33:5: page allocation failure: order:7, mode:0xc04(GFP_NOIO|GFP_DMA32), nodemask=(null),cpuset=/,mems_allowed=0
> > > >
> > > > Just to be clear, this is not a page cache problem.  The driver is asking
> > > > us to do a 512kB allocation without doing I/O!  This is a ridiculous
> > > > request that should be expected to fail.
> > > >
> > > > The solution, whatever it may be, is not related to the page cache.
> > > > I reject your diagnosis.  Almost all of the page cache is clean and
> > > > could be dropped (as far as I can tell from the output below).
> > > >
> > > > Now, I'm not too familiar with how the page allocator chooses to fail
> > > > this request.  Maybe it should be trying harder to drop bits of the page
> > > > cache.  Maybe it should be doing some compaction.
> > That's very thoughtful. I'll look at the page allocator why isn't it dropping
> > cache or doing compaction.
> >
> > > > I am not inclined to
> > > > go digging on your behalf, because frankly I'm offended by the suggestion
> > > > that the page cache is at fault.
> > I apologize—that wasn't my intention.
> >
> > > > Perhaps somebody else will help you, or you can dig into this yourself.
> > >
> > > I'm with Matthew, this really looks like a driver bug somehow.  If there
> > > is page cache memory that is "clean", the driver should be able to
> > > access it just fine if really required.
> > >
> > > What exact driver(s) is having this problem?  What is the exact error,
> > > and on what lines of code?
> > The issue occurs on both ath11k and mhi drivers during resume, when
> > dma_alloc_coherent(GFP_KERNEL) fails and returns -ENOMEM. This failure has
> > been observed at multiple points in these drivers.
> >
> > For example, in the mhi driver, the failure is triggered when the
> > MHI's st_worker gets scheduled-in at resume.
> >
> > mhi_pm_st_worker()
> > -> mhi_fw_load_handler()
> >     -> mhi_load_image_bhi()
> >        -> mhi_alloc_bhi_buffer()
> >           -> dma_alloc_coherent(GFP_KERNEL) returns -ENOMEM
>
> And what is the exact size you are asking for here?
> What is the dma ops set to for your system?  Are you sure that is
> working properly for your platform?  What platform is this exactly?
>
> The driver isn't asking for DMA32 here, so that shouldn't be the issue,
> so why do you feel it is?  Have you tried using the tracing stuff for
> dma allocations to see exactly what is going on for this failure?

I'm guessing the device has a 32-bit DMA mask, and the allocation ends 
up in __dma_direct_alloc_pages() such that that adds GFP_DMA32 in order 
to try to satisfy the mask via regular page allocation. How GFP_KERNEL 
turns into GFP_NOIO, though, given that the DMA layer certainly isn't 
(knowingly) messing with __GFP_IO or __GFP_FS, is more of a mystery... I 
suppose "during resume" is the red flag there - is this worker perhaps 
trying to run too early in some restricted context before the rest of 
the system has fully woken up?

Thanks,
Robin.

> I think you need to do a bit more debugging :)
>
> thanks,
>
> greg k-h