Hi Andrey, Sorry, I got distracted by the merging window. On Tue, May 31, 2022 at 07:16:03PM +0200, Andrey Konovalov wrote: > On Thu, May 26, 2022 at 2:24 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > If we skip unpoisoning (not just poisoning as we already do) for user > > pages, we should reset the tags in page->flags. Whether __GFP_ZEROTAGS > > is passed is complementary, depending on the reason for allocation. > > [...] > > > Currently if __GFP_ZEROTAGS is passed, the unpoisoning is skipped but I > > think we should have just added __GFP_SKIP_KASAN_UNPOISON instead and > > not add a new argument to should_skip_kasan_unpoison(). If we decide to > > always skip unpoisoning, something like below on top of the vanilla > > kernel: > > [...] > > > With the above, we can wire up page_kasan_tag_reset() to the > > __GFP_SKIP_KASAN_UNPOISON check without any additional flags. > > This would make __GFP_SKIP_KASAN_UNPOISON do two logically unrelated > things: skip setting memory tags and reset page tags. This seems > weird. Not entirely weird, it depends on how you look at it. After allocation, you expect the accesses to page_address() to work, irrespective of the GFP flags. __kasan_unpoison_pages() ensures that the page->flags match the written tag without a new GFP flag to set the page->flags. If you skip the unpoisoning something should reset the page->flags tag to ensure an accessible page_address(). I find it weirder that you need another GFP flag to pretty much say 'give me an accessible page'. > I think it makes more sense to split __GFP_ZEROTAGS into > __GFP_ZERO_MEMORY_TAGS and __GFP_ZERO_PAGE_TAGS: the first one does > tag_clear_highpage() without page_kasan_tag_reset() and the second one > does page_kasan_tag_reset() in post_alloc_hook(). Then, add > __GFP_ZERO_PAGE_TAGS to GFP_HIGHUSER_MOVABLE along with > __GFP_SKIP_KASAN_UNPOISON and __GFP_SKIP_KASAN_POISON. And replace > __GFP_ZEROTAGS with __GFP_ZERO_MEMORY_TAGS in > alloc_zeroed_user_highpage_movable(). As above, my preference would be to avoid a new flag, just wire this up to __GFP_SKIP_KASAN_UNPOISON. But if you do want fine-grained control, I can add the above. Thanks. -- Catalin
