On 20.06.22 10:44, Oscar Salvador wrote: > On Mon, Jun 20, 2022 at 04:29:11PM +0800, Muchun Song wrote: >>>> Although it works, I think PageVmemmapSelfHosted() check for the 1st pfn's >>>> vmemmap page is not always reliable. Since we reused PG_owner_priv_1 >>>> as PG_vmemmap_self_hosted, the test is noly reliable for vmemmap page's >>>> vmemmap page. Other non-vmemmap page can be flagged with PG_owner_priv_1. >>>> So this check can be false-positive. Maybe the following code snippet is >>>> the solution. >>> >>> How could that happen for pages used for backing a vmemmap? >>> >> >> It cannot happen for memmap_on_memory case. Howwver, it can happen for other >> cases. E.g. the 1st pfn (of boot memory block) whose vmemmap page may be flagged >> as PG_owner_priv_1 (if PG_swapcache is set). Then, the check is false-positive. > > If this can really happen, which I am not that sure tbh, maybe a way out would be > to just define a new page-type as we did in previous versions of memmap_on_memory. > In that way we would not for flags, but for its type. > > But as I said, I am not entirely sure about the potential fallout of what you mention. We are talking about the memmap of a page, who's page content is the memmap of pages actually exposed to other users (file-backed, anonymous, whatsoever). In other words, while setting PG_swapcache on the memmap of a page exposed to the user is possible, it shouldn't be possible for the memmap of the page "hosting" these memmaps. I know, it's confusing and I keep confusing myself. I tried creating a picture and it doesn't really clarify the situation :D -- Thanks, David / dhildenb
