Hi, On Fri, Jul 16, 2021 at 02:09:58AM -0400, Peilin Ye wrote: > This information is out-of-date, and it took me quite some time of > ftrace'ing before I figured it out... I think it would be beneficial to > update, or at least remove it. > > As a proof that I understand what I am talking about, on my x86_64 box: > > 1. I allocated a vmalloc() area containing linear address `addr`; > 2. I manually pagewalked `addr` in different page tables, including > `init_mm.pgd`; > 3. The corresponding PGD entries for `addr` in different page tables, > they all immediately pointed at the same PUD table (my box uses > 4-level paging), at the same physical address; > 4. No "lazy synchronization" via page fault handling happened at all, > since it is the same PUD table pre-allocated by > preallocate_vmalloc_pages() during boot time. Yes, this is the story for x86-64, because all PUD/P4D pages for the vmalloc area are pre-allocated at boot. So no faulting or synchronization needs to happen. On x86-32 this is a bit different. Pre-allocation of PMD/PTE pages is not an option there (even less when 4MB large-pages with 2-level paging come into the picture). So what happens there is that vmalloc related changes to the init_mm.pgd are synchronized to all page-tables in the system. But this synchronization is subject to race conditions in a way that another CPU might vmalloc an area below a PMD which is not fully synchronized yet. When this happens there is a fault, which is handled as a vmalloc() fault on x86-32 just as before. So vmalloc faults still exist on 32-bit, they are just less likely as they used to be. Regards, Joerg
