Hi Dan, On Tue, Jan 31, 2017 at 08:09:53AM -0500, Dan Streetman wrote: > On Mon, Jan 30, 2017 at 7:10 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote: > > Hi Dan, > > > > On Thu, Jan 26, 2017 at 12:04:03PM -0500, Dan Streetman wrote: > >> On Wed, Jan 25, 2017 at 12:26 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote: > >> > On Tue, Jan 24, 2017 at 11:06:51PM -0500, Chulmin Kim wrote: > >> >> On 01/23/2017 12:40 AM, Minchan Kim wrote: > >> >> >On Mon, Jan 23, 2017 at 02:30:56PM +0900, Sergey Senozhatsky wrote: > >> >> >>On (01/23/17 14:22), Minchan Kim wrote: > >> >> >>[..] > >> >> >>>>Anyway, I will let you know the situation when it gets more clear. > >> >> >>> > >> >> >>>Yeb, Thanks. > >> >> >>> > >> >> >>>Perhaps, did you tried flush page before the writing? > >> >> >>>I think arm64 have no d-cache alising problem but worth to try it. > >> >> >>>Who knows :) > >> >> >> > >> >> >>I thought that flush_dcache_page() is only for cases when we write > >> >> >>to page (store that makes pages dirty), isn't it? > >> >> > > >> >> >I think we need both because to see recent stores done by the user. > >> >> >I'm not sure it should be done by block device driver rather than > >> >> >page cache. Anyway, brd added it so worth to try it, I thought. :) > >> >> > > >> >> > >> >> Thanks for the suggestion! > >> >> It might be helpful > >> >> though proving it is not easy as the problem appears rarely. > >> >> > >> >> Have you thought about > >> >> zram swap or zswap dealing with self modifying code pages (ex. JIT)? > >> >> (arm64 may have i-cache aliasing problem) > >> > > >> > It can happen, I think, although I don't know how arm64 handles it. > >> > > >> >> > >> >> If it is problematic, > >> >> especiallly zswap (without flush_dcache_page in zswap_frontswap_load()) may > >> >> provide the corrupted data > >> >> and even swap out (compressing) may see the corrupted data sooner or later, > >> >> i guess. > >> > > >> > try_to_unmap_one calls flush_cache_page which I hope to handle swap-out side > >> > but for swap-in, I think zswap need flushing logic because it's first > >> > touch of the user buffer so it's his resposibility. > >> > >> Hmm, I don't think zswap needs to, because all the cache aliases were > >> flushed when the page was written out. After that, any access to the > >> page will cause a fault, and the fault will cause the page to be read > >> back in (via zswap). I don't see how the page could be cached at any > >> time between the swap write-out and swap read-in, so there should be > >> no need to flush any caches when it's read back in; am I missing > >> something? > > > > Documentation/cachetlb.txt says > > > > void flush_dcache_page(struct page *page) > > > > Any time the kernel writes to a page cache page, _OR_ > > the kernel is about to read from a page cache page and > > user space shared/writable mappings of this page potentially > > exist, this routine is called. > > > > For swap-in side, I don't see any logic to prevent the aliasing > > problem. Let's consider other examples like cow_user_page-> > > copy_user_highpage. For architectures which can make aliasing, > > it has arch specific functions which has flushing function. > > COW works with a page that has a physical backing. swap-in does not. > COW pages can be accessed normally; swapped out pages cannot. > > > > > IOW, if a kernel makes store operation to the page which will > > be mapped to user space address, kernel should call flush function. > > Otherwise, user space will miss recent update from kernel side. > > as I said before, when it's swapped out caches are flushed, and the > page mapping invalidated, so it will cause a fault on any access, and > thus cause swap to re-load the page from disk (or zswap). So how > would a cache of the page be created after swap-out, but before > swap-in? It's not possible for user space to have any caches to the > page, unless (as I said) I'm missing something? > Let's assume VIVT architecture which get index and tag from virtual address. In zswap_frontswap_load, let's assume dst is kernel virtual address, 0xc0002000 so cacheline for 0xc0002000 has recent uptodate data. Now, VM mapped the page into user address space, 0x80003000, for example. In that case, userland application try to read data from 0x80003000 but it is associated with another cacheline so it cannot see recent uptodate data written by zswap. flush_dcache_page will handle both kernel side and user side. I'm not sure how I explain well. :-( Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
