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a=XiAAW1AwiKB2Y8Wsi+sD2Q==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=kj9zAlcOel0A:10 a=2OwXVqhp2XgA:10 a=GoEa3M9JfhUA:10 a=yPCof4ZbAAAA:8 a=8Z0Aofs3deCmuYkeYmMA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwODIzMDAzMCBTYWx0ZWRfX7eF2YGJI6Cn8 l/x9fP0C0Fkq7UD8jjOyT9Oh6n1Nw3u9bSwTUnDpdA7DqhO0X8jXC5FNkE9aZcUi2fNcoUpJ0iy xvPBG7CEiRb6hVbCBOQjXEqSE3ZQ5NXfvfaTzMCpScWHzz07LD2k9OypT5YthMCNTwi/XXF7ZRp jZwB2opw8PqQGvelq92PvtUd1T/CYseNueY2QOm7un+hC4+FrFhvCtokLS1Gb3WYUb/FzPTybyy zCdoX9dhxBM1yC4Lg0rxtvr3XbE273rj4/dGb5Uif3RmekUrxa6L+FWIFKHPk/oO+9NXtJc3M2A E54dqNCUSAJLxkdrNwSaQvFaT4I6EJFKDfm7dRXBuBvs5YtgB4t9oQTi9feqBmAe98HLfVZTlxf JuePvHCi On Tue, Aug 26, 2025 at 03:58:48PM +0900, Harry Yoo wrote: > While move_ptes() explains when rmap locks can be skipped, when reading > the code setting pmc.need_rmap_locks it is not immediately obvious when > need_rmap_locks can be false. Add a brief explanation in copy_vma() and > relocate_vma_down(), and add a pointer to the comment in move_ptes(). > > Meanwhile, fix and improve the comment in move_ptes(). > > Signed-off-by: Harry Yoo This is great thanks! :) > --- > mm/mremap.c | 4 +++- > mm/vma.c | 7 +++++++ > mm/vma_exec.c | 5 +++++ > 3 files changed, 15 insertions(+), 1 deletion(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index e618a706aff5..86adb872bea0 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -218,8 +218,10 @@ static int move_ptes(struct pagetable_move_control *pmc, > * When need_rmap_locks is false, we use other ways to avoid > * such races: > * > - * - During exec() shift_arg_pages(), we use a specially tagged vma > + * - During exec() relocate_vma_down(), we use a specially tagged vma > * which rmap call sites look for using vma_is_temporary_stack(). > + * Folios mapped in the temporary stack vma cannot be migrated until > + * the relocation is complete. Can we actually move this comment over to move_page_tables()? As this is relevant to the whole operation. Also could you put a comment referencing this comment in copy_vma_and_data() as this is where we actually determine whether this is the case or not in _most cases_. Let's just get all the 'need rmap locks' and 'corner cases where it's fine anyway' in one place that is logical :) Also could you put a comment in copy_vma() over in mm/vma.c saying 'see the comment in mm/mremap.c' or even risk mentioning the function name (risky as code changes but still :P) e.g. 'see comment in move_page_tables()' or something. I'm confused by the 'folios mapped' and 'migrate' bits - and I think people will be confused by that. I think better to say 'page tables for the temporary stack cannot be adjusted until the relocation is complete'. > * > * - During mremap(), new_vma is often known to be placed after vma > * in rmap traversal order. This ensures rmap will always observe This whole bit after could really do with some ASCII diagrams btw :)) ;) but you know maybe out of scope here. > diff --git a/mm/vma.c b/mm/vma.c > index 3b12c7579831..3da49f79e9ba 100644 > --- a/mm/vma.c > +++ b/mm/vma.c > @@ -1842,6 +1842,11 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap, > vmg.next = vma_iter_next_rewind(&vmi, NULL); > new_vma = vma_merge_new_range(&vmg); > > + /* > + * rmap locks can be skipped as long as new_vma is traversed > + * after vma during rmap walk (new_vma->vm_pgoff >= vma->vm_pgoff). > + * See the comment in move_ptes(). > + */ Obv. would prefer this to say 'move_page_tables()' as mentioned above :P > if (new_vma) { > /* > * Source vma may have been merged into new_vma > @@ -1879,6 +1884,8 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap, > new_vma->vm_ops->open(new_vma); > if (vma_link(mm, new_vma)) > goto out_vma_link; > + > + /* new_vma->pg_off is always >= vma->pg_off if not merged */ Err, new_vma is NULL? :) I'm not sure this comment is too useful. > *need_rmap_locks = false; > } > return new_vma; > diff --git a/mm/vma_exec.c b/mm/vma_exec.c > index 922ee51747a6..a895dd39ac46 100644 > --- a/mm/vma_exec.c > +++ b/mm/vma_exec.c > @@ -63,6 +63,11 @@ int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift) > * process cleanup to remove whatever mess we made. > */ > pmc.for_stack = true; > + /* > + * pmc.need_rmap_locks is false since rmap locks can be safely skipped > + * because migration is disabled for this vma during relocation. > + * See the comment in move_ptes(). > + */ Let's reword this also, people will get confused about migration here. 'pmc.need_rmap_locks is false since rmap explicitly checks for vma_is_temporary_stack() and thus extra care does not need to be taken here during stack relocation. See the comment in move_page_tables().' > if (length != move_page_tables(&pmc)) > return -ENOMEM; > > -- > 2.43.0 > Cheers, Lorenzo