Am 30.01.19 um 09:02 schrieb Christoph Hellwig: > On Tue, Jan 29, 2019 at 08:58:35PM +0000, Jason Gunthorpe wrote: >> On Tue, Jan 29, 2019 at 01:39:49PM -0700, Logan Gunthorpe wrote: >> >>> implement the mapping. And I don't think we should have 'special' vma's >>> for this (though we may need something to ensure we don't get mapping >>> requests mixed with different types of pages...). >> I think Jerome explained the point here is to have a 'special vma' >> rather than a 'special struct page' as, really, we don't need a >> struct page at all to make this work. >> >> If I recall your earlier attempts at adding struct page for BAR >> memory, it ran aground on issues related to O_DIRECT/sgls, etc, etc. > Struct page is what makes O_DIRECT work, using sgls or biovecs, etc on > it work. Without struct page none of the above can work at all. That > is why we use struct page for backing BARs in the existing P2P code. > Not that I'm a particular fan of creating struct page for this device > memory, but without major invasive surgery to large parts of the kernel > it is the only way to make it work. The problem seems to be that struct page does two things: 1. Memory management for system memory. 2. The object to work with in the I/O layer. This was done because a good part of that stuff overlaps, like reference counting how often a page is used. The problem now is that this doesn't work very well for device memory in some cases. For example on GPUs you usually have a large amount of memory which is not even accessible by the CPU. In other words you can't easily create a struct page for it because you can't reference it with a physical CPU address. Maybe struct page should be split up into smaller structures? I mean it's really overloaded with data. Christian.
