On Mon, Jun 20, 2022 at 10:44:40AM +0200, 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 I need to clarify this only can be happened by using this approach implemented in this patch. For a boot memory block, the vemmmap pages are not slef-hosted. So the 1st pfn (of this memory block) can be allocated to other users. e.g. an anonymous page with PG_swapcache set. In this patch, ALIGN_DOWN(pfn, PHYS_PFN(memory_block_size_bytes())) will located on this anonymous page, then the check is false-positive. [ boot memory block ] [ section ][...][ section ] [ usable memory ] > 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. > I think we do not need to introduced a new flag, we just make sure the page passed to PageVmemmapSelfHosted() is a backing page for vmemmap. Then we cannot incur false-positive. The feasible solution is walking page tables to find a vmemmap page's backing page. Thanks. > But as I said, I am not entirely sure about the potential fallout of what you mention. > > > -- > Oscar Salvador > SUSE Labs >
