From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> Subject: XArray: add xa_get_order Patch series "Fix read-only THP for non-tmpfs filesystems". As described more verbosely in the [3/3] changelog, we can inadvertently put an order-0 page in the page cache which occupies 512 consecutive entries. Users are running into this if they enable the READ_ONLY_THP_FOR_FS config option; see https://bugzilla.kernel.org/show_bug.cgi?id=206569 and Qian Cai has also reported it here: https://lore.kernel.org/lkml/20200616013309.GB815@xxxxxx/ This is a rather intrusive way of fixing the problem, but has the advantage that I've actually been testing it with the THP patches, which means that it sees far more use than it does upstream -- indeed, Song has been entirely unable to reproduce it. It also has the advantage that it removes a few patches from my gargantuan backlog of THP patches. This patch (of 3): This function returns the order of the entry at the index. We need this because there isn't space in the shadow entry to encode its order. [akpm@xxxxxxxxxxxxxxxxxxxx: export xa_get_order to modules] Link: https://lkml.kernel.org/r/20200903183029.14930-1-willy@xxxxxxxxxxxxx Link: https://lkml.kernel.org/r/20200903183029.14930-2-willy@xxxxxxxxxxxxx Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Cc: "Kirill A . Shutemov" <kirill@xxxxxxxxxxxxx> Cc: Qian Cai <cai@xxxxxx> Cc: Song Liu <songliubraving@xxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/xarray.h | 9 ++++++++ lib/test_xarray.c | 21 ++++++++++++++++++++ lib/xarray.c | 40 +++++++++++++++++++++++++++++++++++++++ 3 files changed, 70 insertions(+) --- a/include/linux/xarray.h~xarray-add-xa_get_order +++ a/include/linux/xarray.h @@ -1505,6 +1505,15 @@ void xas_pause(struct xa_state *); void xas_create_range(struct xa_state *); +#ifdef CONFIG_XARRAY_MULTI +int xa_get_order(struct xarray *, unsigned long index); +#else +static inline int xa_get_order(struct xarray *xa, unsigned long index) +{ + return 0; +} +#endif + /** * xas_reload() - Refetch an entry from the xarray. * @xas: XArray operation state. --- a/lib/test_xarray.c~xarray-add-xa_get_order +++ a/lib/test_xarray.c @@ -1649,6 +1649,26 @@ static noinline void check_account(struc #endif } +static noinline void check_get_order(struct xarray *xa) +{ + unsigned int max_order = IS_ENABLED(CONFIG_XARRAY_MULTI) ? 20 : 1; + unsigned int order; + unsigned long i, j; + + for (i = 0; i < 3; i++) + XA_BUG_ON(xa, xa_get_order(xa, i) != 0); + + for (order = 0; order < max_order; order++) { + for (i = 0; i < 10; i++) { + xa_store_order(xa, i << order, order, + xa_mk_index(i << order), GFP_KERNEL); + for (j = i << order; j < (i + 1) << order; j++) + XA_BUG_ON(xa, xa_get_order(xa, j) != order); + xa_erase(xa, i << order); + } + } +} + static noinline void check_destroy(struct xarray *xa) { unsigned long index; @@ -1697,6 +1717,7 @@ static int xarray_checks(void) check_reserve(&array); check_reserve(&xa0); check_multi_store(&array); + check_get_order(&array); check_xa_alloc(); check_find(&array); check_find_entry(&array); --- a/lib/xarray.c~xarray-add-xa_get_order +++ a/lib/xarray.c @@ -1592,6 +1592,46 @@ unlock: return xas_result(&xas, NULL); } EXPORT_SYMBOL(xa_store_range); + +/** + * xa_get_order() - Get the order of an entry. + * @xa: XArray. + * @index: Index of the entry. + * + * Return: A number between 0 and 63 indicating the order of the entry. + */ +int xa_get_order(struct xarray *xa, unsigned long index) +{ + XA_STATE(xas, xa, index); + void *entry; + int order = 0; + + rcu_read_lock(); + entry = xas_load(&xas); + + if (!entry) + goto unlock; + + if (!xas.xa_node) + goto unlock; + + for (;;) { + unsigned int slot = xas.xa_offset + (1 << order); + + if (slot >= XA_CHUNK_SIZE) + break; + if (!xa_is_sibling(xas.xa_node->slots[slot])) + break; + order++; + } + + order += xas.xa_node->shift; +unlock: + rcu_read_unlock(); + + return order; +} +EXPORT_SYMBOL(xa_get_order); #endif /* CONFIG_XARRAY_MULTI */ /** _
