Messages in this thread | | | From | "Huang, Ying" <> | Subject | Re: Data corruption problem with swapfiles and THP | Date | Fri, 13 Aug 2021 08:21:38 +0800 |
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Matthew Wilcox <willy@infradead.org> writes:
> There is an assumption in the swap writepage path that a THP is physically > contiguous on swap: > > bio->bi_iter.bi_sector = swap_page_sector(page); > bio->bi_opf = REQ_OP_WRITE | REQ_SWAP | wbc_to_write_flags(wbc); > bio->bi_end_io = end_write_func; > bio_add_page(bio, page, thp_size(page), 0); > > As far as I can tell, this is not necessarily true. If a file is not > contiguous, we can have an extent which is 1MB long followed by an extent > somewhere else on storage that's 1MB long. When we try to write a 2MB > page to swap, we overwrite whatever's on the block device after that > first 1MB extent. > > (Came across this by code examination while looking at getting rid of > the bio path entirely; no attempt has been made to produce this problem; > something else may prevent it from actually happening)
Yes. THP needs to be split firstly before swapping out to a swap device backed by a file. Please take a look at the get_swap_pages()
if (size == SWAPFILE_CLUSTER) { if (si->flags & SWP_BLKDEV) n_ret = swap_alloc_cluster(si, swp_entries); } else n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE, n_goal, swp_entries);
If the swap device is backed by a file, si->flags & SWP_BLKDEV == 0, only normal swap entry (not huge) can be allocated. This will result that the THP is split.
Best Regards, Huang, Ying
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