On Thu, Apr 06, 2023 at 05:51:52PM +0200, David Hildenbrand wrote: > On 06.04.23 17:02, Peter Zijlstra wrote: > > DavidH, what do you thikn about reviving Jann's patches here: > > > > https://bugs.chromium.org/p/project-zero/issues/detail?id=2365#c1 > > > > Those are far more invasive, but afaict they seem to do the right thing. > > > > I recall seeing those while discussed on security@xxxxxxxxxx. What we > currently have was (IMHO for good reasons) deemed better to fix the issue, > especially when caring about backports and getting it right. Yes, and I think that was the right call. However, we can now revisit without having the pressure of a known defect and backport considerations. > The alternative that was discussed in that context IIRC was to simply > allocate a fresh page table, place the fresh page table into the list > instead, and simply free the old page table (then using common machinery). > > TBH, I'd wish (and recently raised) that we could just stop wasting memory > on page tables for THPs that are maybe never going to get PTE-mapped ... and > eventually just allocate on demand (with some caching?) and handle the > places where we're OOM and cannot PTE-map a THP in some descend way. > > ... instead of trying to figure out how to deal with these page tables we > cannot free but have to special-case simply because of GUP-fast. Not keeping them around sounds good to me, but I'm not *that* familiar with the THP code, most of that happened after I stopped tracking mm. So I'm not sure how feasible is it. But it does look entirely feasible to rework this page-table freeing along the lines Jann did.
