On Tue, Sep 22, 2020 at 06:53:55PM +0200, Oleg Nesterov wrote: > On 09/22, Peter Xu wrote: > > > > On Tue, Sep 22, 2020 at 05:48:46PM +0200, Oleg Nesterov wrote: > > > > However since I didn't change this logic in this patch, it probably > > > > means this > > > > bug is also in the original code before this series... I'm thinking > > > > maybe I > > > > should prepare a standalone patch to clear the swp_entry_t and cc > > > > stable. > > > > > > Well, if copy_one_pte(src_pte) hits a swap entry and returns entry.val != > > > 0, then > > > pte_none(*src_pte) is not possible after restart? This means that > > > copy_one_pte() > > > will be called at least once. > > > > Note that we've released the page table locks, so afaict the old swp entry > > can > > be gone under us when we go back to the "do" loop... :) > > But how? > > I am just curious, I don't understand this code enough.
Me neither. The point is I think we can't assume *src_pte will read the same if we have released the src_ptl in copy_pte_range(), because imho the src_ptl is the only thing to protect it. Or to be more explicit, we need pte_alloc_map_lock() to read a stable pmd/pte or before update (since src_ptl itself could change too). Thanks, -- Peter Xu
