From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <452A35FF.50009@tungstengraphics.com> Date: Mon, 09 Oct 2006 13:43:59 +0200 From: =?ISO-8859-1?Q?Thomas_Hellstr=F6m?= MIME-Version: 1.0 Subject: Re: User switchable HW mappings & cie References: <1160347065.5926.52.camel@localhost.localdomain> In-Reply-To: <1160347065.5926.52.camel@localhost.localdomain> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Benjamin Herrenschmidt Cc: linux-mm@kvack.org, Linux Kernel list , Hugh Dickins , Arnd Bergmann , Linus Torvalds List-ID: Benjamin Herrenschmidt wrote: >Hi ! > >I'd like to kick a discussion about some issues I've been having along >with some proposed solutions related to mapping of bits of hardware in >smarter ways than simply doing a io_remap_pfn_range() and the problems, >generally caused by get_user_pages(). > > > ... >The Tungstengrpahics folks (Thomas is on CC) have been working on some >better memory management to properly handle those things in the DRM. One >of the things we want to do here is similar to what the SPUs do with >local store: have a single VMA associated with an object, and have the >PTEs transparently changed to map either video memory, system memory, >AGP memory, etc... (the different in cache attributes can be ignored at >this stage, we can discuss it separately if interested). > >I've been suggesting a similar approach as we use for SPUs. That is what >would make the most sense from a user standpoint: user code access their >"objects" via a single virtual pointer and the DRM takes care of >migrating it when necessary (for example migrating it to video RAM when >it needs to be used by the engine and "swap" it back to main memory when >not). > > > > ... >The base idea is that we would have the no_page() function of SPU's or >the DRM either return a struct page when the object is backed to main >memory, or install the PTE directly (using the helper to hide some of >the low level TLB flushing logic etc...) and then return NOPAGE_REFAULT >when hitting the hardware. The helper basically is a one-page version of >io_remap_pfn_range() with the added "feature" of not doing anything if >the PTE has been set by somebody else (handle the race case) instead of >BUG'ing as the current io_remap_pfn_range() does. > > > I'm very much for this approach, possibly with the extension that we could have a multiple-page version as well, as populating the whole vma sometimes may be cheaper than populating each pte with a fault. That would basically be an io_remap_pfn_range() which is safe when the mmap_sem is taken in read mode (from do_no_page). One problem that occurs is that the rule for ptes with non-backing struct pages Which I think was introduced in 2.6.16: pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT) cannot be honored, at least not with the DRM memory manager, since the graphics object will be associated with a vma and not the underlying physical address. User space will have vma->vm_pgoff as a handle to the object, which may move around in graphics memory. /Thomas -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org