On Fri, 03 May 2019 19:00:26 -0500, Probir Roy said: > > > While searching for next slot in a hole, it walks through the same > > > slots over n over. > > > > How did you determine this? > > I am working on a tool that identifies repeated load of an address. > Often these repeated loads are redundant and can be avoided with data > structure modification. The tool points me to this line. Is this doing static analysis, or actually doing run-time tracing? > > Looks to me like the ++offset will walk through each potential slot once, > > and break out if it finds one. > > > This function is being called by the radix_tree_for_each_slot > iterator, defined as follows: > > #define radix_tree_for_each_slot(slot, root, iter, start) \ > for (slot = radix_tree_iter_init(iter, start) ; \ > slot || (slot = radix_tree_next_chunk(root, iter, 0)) ; \ > // <<<<-------^^^ > slot = radix_tree_next_slot(slot, iter, 0)) > > Here is the calling context I get: > |_ depth: 1 :0, method: ext4_block_write_begin+0x335/0x4f0(), > |_ depth: 2 :0, method: alloc_buffer_head+0x21/0x60(), > |_ depth: 3 :0, method: ext4_da_get_block_prep+0x1a6/0x490(), > |_ depth: 4 :0, method: clean_bdev_aliases+0x9a/0x210(), > |_ depth: 5 :0, method: pagevec_lookup_range+0x24/0x30(), > |_ depth: 6 :0, method: find_get_pages_range+0x151/0x2d0(), > |_ depth: 7 :0, method: radix_tree_next_chunk+0x10f/0x360() > > Does it explain the case? Actually, that calling context doesn't tell us much of anything till depth 7. Yes, next_chunk() and next_slot() can get called repeatedly, especially if it's a large radix tree. The important question is: Is it being called with the *same value* of 'slot' repeatedly? Looking at the code, it's pretty obvious that 'slot' will be updated at least once through every pass through the for_each_slot(), unless the radix tree is corrupted. If you're trying to do static analysis, your code may be confused by either the 'slot || next_chunk()' iterator, or the fact that 'slot' is assigned both in the for loops iterator and in the body of the loop, and thus failing to detect that slot is updated.
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