Messages in this thread | | | Subject | Re: [v2 PATCH] mm: move_pages: return valid node id in status if the page is already on the target node | From | Yang Shi <> | Date | Thu, 5 Dec 2019 09:20:10 -0800 |
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On 12/4/19 9:44 PM, John Hubbard wrote: > On 12/4/19 8:21 PM, Yang Shi wrote: >> Felix Abecassis reports move_pages() would return random status if the >> pages are already on the target node by the below test program: >> >> ---8<--- > > This is correct correct code, so: > > Reviewed-by: John Hubbard <jhubbard@nvidia.com> > > ...with a few nitpicky notes about comments, below, that might help:
Thanks, John. Will take in new version.
> >> >> int main(void) >> { >> const long node_id = 1; >> const long page_size = sysconf(_SC_PAGESIZE); >> const int64_t num_pages = 8; >> >> unsigned long nodemask = 1 << node_id; >> long ret = set_mempolicy(MPOL_BIND, &nodemask, sizeof(nodemask)); >> if (ret < 0) >> return (EXIT_FAILURE); >> >> void **pages = malloc(sizeof(void*) * num_pages); >> for (int i = 0; i < num_pages; ++i) { >> pages[i] = mmap(NULL, page_size, PROT_WRITE | PROT_READ, >> MAP_PRIVATE | MAP_POPULATE | MAP_ANONYMOUS, >> -1, 0); >> if (pages[i] == MAP_FAILED) >> return (EXIT_FAILURE); >> } >> >> ret = set_mempolicy(MPOL_DEFAULT, NULL, 0); >> if (ret < 0) >> return (EXIT_FAILURE); >> >> int *nodes = malloc(sizeof(int) * num_pages); >> int *status = malloc(sizeof(int) * num_pages); >> for (int i = 0; i < num_pages; ++i) { >> nodes[i] = node_id; >> status[i] = 0xd0; /* simulate garbage values */ >> } >> >> ret = move_pages(0, num_pages, pages, nodes, status, MPOL_MF_MOVE); >> printf("move_pages: %ld\n", ret); >> for (int i = 0; i < num_pages; ++i) >> printf("status[%d] = %d\n", i, status[i]); >> } >> ---8<--- >> >> Then running the program would return nonsense status values: >> $ ./move_pages_bug >> move_pages: 0 >> status[0] = 208 >> status[1] = 208 >> status[2] = 208 >> status[3] = 208 >> status[4] = 208 >> status[5] = 208 >> status[6] = 208 >> status[7] = 208 >> >> This is because the status is not set if the page is already on the >> target node, but move_pages() should return valid status as long as it >> succeeds. The valid status may be errno or node id. >> >> We can't simply initialize status array to zero since the pages may be >> not on node 0. Fix it by updating status with node id which the page is >> already on. And, it looks we have to update the status inside >> add_page_for_migration() since the page struct is not available outside >> it. >> >> Make add_page_for_migration() return 1 if store_status() is failed in >> order to not mix up the status value since -EFAULT is also a valid >> status. >> >> Fixes: a49bd4d71637 ("mm, numa: rework do_pages_move") >> Reported-by: Felix Abecassis <fabecassis@nvidia.com> >> Tested-by: Felix Abecassis <fabecassis@nvidia.com> >> Cc: John Hubbard <jhubbard@nvidia.com> >> Cc: Michal Hocko <mhocko@suse.com> >> Cc: Christoph Lameter <cl@linux.com> >> Cc: Vlastimil Babka <vbabka@suse.cz> >> Cc: Mel Gorman <mgorman@techsingularity.net> >> Cc: <stable@vger.kernel.org> 4.17+ >> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> >> --- >> v2: *Correted the return value when add_page_for_migration() returns 1. >> >> John noticed another return value inconsistency between the >> implementation and >> the manpage. The manpage says it should return -ENOENT if the page >> is already >> on the target node, but it doesn't. It looks the original code >> didn't return >> -ENOENT either, I'm not sure if this is a document issue or not. >> Anyway this >> is another issue, once we confirm it we can fix it later. >> >> mm/migrate.c | 36 ++++++++++++++++++++++++++++++------ >> 1 file changed, 30 insertions(+), 6 deletions(-) >> >> diff --git a/mm/migrate.c b/mm/migrate.c >> index a8f87cb..f1090a0 100644 >> --- a/mm/migrate.c >> +++ b/mm/migrate.c >> @@ -1512,17 +1512,21 @@ static int do_move_pages_to_node(struct >> mm_struct *mm, >> /* >> * Resolves the given address to a struct page, isolates it from >> the LRU and >> * puts it to the given pagelist. >> - * Returns -errno if the page cannot be found/isolated or 0 when it >> has been >> - * queued or the page doesn't need to be migrated because it is >> already on >> - * the target node >> + * Returns: >> + * errno - if the page cannot be found/isolated >> + * 0 - when it has been queued or the page doesn't need to be >> migrated >> + * because it is already on the target node >> + * 1 - if store_status() is failed > > > I recommend this wording instead: > > * Returns: > * errno - if the page cannot be found/isolated > * 0 - when it has been queued or the page doesn't need to be > migrated > * because it is already on the target node > * 1 - The page doesn't need to be migrated because it is already > on the > * target node. However, attempting to store the node ID in > the status > * array failed. Unlike other failures in this function, this > case > * needs to turn into a fatal failure in the calling function. > > >> */ >> static int add_page_for_migration(struct mm_struct *mm, unsigned >> long addr, >> - int node, struct list_head *pagelist, bool migrate_all) >> + int node, struct list_head *pagelist, bool migrate_all, >> + int __user *status, int start) >> { >> struct vm_area_struct *vma; >> struct page *page; >> unsigned int follflags; >> int err; >> + bool same_node = false; >> down_read(&mm->mmap_sem); >> err = -EFAULT; >> @@ -1543,8 +1547,10 @@ static int add_page_for_migration(struct >> mm_struct *mm, unsigned long addr, >> goto out; >> err = 0; >> - if (page_to_nid(page) == node) >> + if (page_to_nid(page) == node) { >> + same_node = true; >> goto out_putpage; >> + } >> err = -EACCES; >> if (page_mapcount(page) > 1 && !migrate_all) >> @@ -1578,6 +1584,16 @@ static int add_page_for_migration(struct >> mm_struct *mm, unsigned long addr, >> put_page(page); >> out: >> up_read(&mm->mmap_sem); >> + >> + /* >> + * Must call store_status() after releasing mmap_sem since put_user >> + * need acquire mmap_sem too, otherwise potential deadlock may >> exist. >> + */ >> + if (same_node) { >> + if (store_status(status, start, node, 1)) >> + err = 1; >> + } >> + >> return err; >> } >> @@ -1639,10 +1655,18 @@ static int do_pages_move(struct mm_struct >> *mm, nodemask_t task_nodes, >> * report them via status >> */ > > Let's change the comment above add_page_for_migration(), to read: > > /* > * Most errors in the page lookup or isolation are not fatal > * and we simply report them via the status array. However, > * positive error values are fatal. > */ > > >> err = add_page_for_migration(mm, addr, current_node, >> - &pagelist, flags & MPOL_MF_MOVE_ALL); >> + &pagelist, flags & MPOL_MF_MOVE_ALL, status, >> + i); >> + >> if (!err) >> continue; >> + /* store_status() failed in add_page_for_migration() */ > > ...and let's replace the above line, with the following: > > /* > * Most errors in the page lookup or isolation are not fatal > * and we simply report them via the status array. However, > * positive error values are fatal. > */ > > >> + if (err > 0) { >> + err = -EFAULT; >> + goto out_flush; >> + } >> + >> err = store_status(status, i, err, 1); >> if (err) >> goto out_flush; >> > > And with that, I think the comments help a little bit more, in reading > through the code. > > > thanks,
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