Hi Christoph, On Mon, Apr 19, 2021 at 08:34:41AM +0200, Christoph Hellwig wrote: > On Fri, Apr 16, 2021 at 04:27:55PM +0100, Matthew Wilcox wrote: > > On Thu, Apr 15, 2021 at 08:08:32PM +0200, Jesper Dangaard Brouer wrote: > > > See below patch. Where I swap32 the dma address to satisfy > > > page->compound having bit zero cleared. (It is the simplest fix I could > > > come up with). > > > > I think this is slightly simpler, and as a bonus code that assumes the > > old layout won't compile. > > So, why do we even do this crappy overlay of a dma address? This just > all seems like a giant hack. Random subsystems should not just steal > a few struct page fields as that just turns into the desasters like the > one we've seen here or probably something worse next time.
The page pool API was using page->private in the past to store these kind of info. That caused a problem to begin with, since it would fail on 32-bit systems with 64bit DMA. We had a similar discussion on the past but decided struct page is the right place to store that [1]. Another advantage is that we can now use the information from the networking subsystem and enable recycling of SKBs and SKB fragments, by using the stored metadata of struct page [2]. [1] https://lore.kernel.org/netdev/20190207.132519.1698007650891404763.da...@davemloft.net/ [2] https://lore.kernel.org/netdev/20210409223801.104657-1-mcr...@linux.microsoft.com/ Cheers /Ilias
