On 08/07/2021 12:50 am, Trond Myklebust wrote:
On Thu, 2021-07-08 at 00:13 +0100, Calum Mackay wrote:On 07/07/2021 11:01 pm, Trond Myklebust wrote:On Wed, 2021-07-07 at 19:51 +0100, Calum Mackay wrote:hi Trond, I had a question about these two old commits of yours, from v5.0 & v5.2: 14bebe3c90b3 NFS: Don't interrupt file writeout due to fatal errors (2 years, 2 months ago) 8fc75bed96bb NFS: Fix up return value on fatal errors in nfs_page_async_flush() (2 years, 5 months ago) I am looking at a crash dump, with a kernel based on an older- still v4.14 stable which did not have either of the above commits. PANIC: "BUG: unable to handle kernel NULL pointer dereference at 0000000000000080" [exception RIP: _raw_spin_lock+20] #10 [ffffb1493d78fcb8] nfs_updatepage at ffffffffc08f1791 [nfs] #11 [ffffb1493d78fd10] nfs_write_end at ffffffffc08e094e [nfs] #12 [ffffb1493d78fd58] generic_perform_write at ffffffffa71d458b #13 [ffffb1493d78fde0] nfs_file_write at ffffffffc08dfdb4 [nfs] #14 [ffffb1493d78fe18] __vfs_write at ffffffffa72848bc #15 [ffffb1493d78fea0] vfs_write at ffffffffa7284ad2 #16 [ffffb1493d78fee0] sys_write at ffffffffa7284d35 #17 [ffffb1493d78ff28] do_syscall_64 at ffffffffa7003949 the real sequence, obscured by compiler inlining, is: nfs_updatepage nfs_writepage_setup nfs_setup_write_request nfs_inode_add_request spin_lock(&mapping->private_lock); and we crash since the as mapping pointer is NULL. I thought I was able to construct a possible sequence that would explain the above, if we are in (from above): nfs_setup_write_request nfs_try_to_update_request nfs_wb_page nfs_writepage_locked nfs_do_writepage and nfs_page_async_flush detects a fatal server error, and calls nfs_write_error_remove_page, which results in the page->mapping set to NULL. In that version of the code, without your commits above, nfs_page_async_flush returns 0 in this case, which I thought might result in nfs_setup_write_request going ahead and calling nfs_inode_add_request with that page, resulting in the crash seen. I then discovered your v5.0 commit: 8fc75bed96bb NFS: Fix up return value on fatal errors in nfs_page_async_flush() (2 years, 5 months ago) which appeared to correct that, having nfs_page_async_flush return the error in this case, so we would not end up in nfs_inode_add_request. But I then spotted your later v5.2 commit: 14bebe3c90b3 NFS: Don't interrupt file writeout due to fatal errors (2 years, 2 months ago) which changes things back, so that nfs_page_async_flush now again returns 0, in the "launder" case, and that's how that code remains today. If so, is there anything to stop the possible crash path that I describe above? path I suggest above? Or perhaps I'm missing another commit that stops it happening, even after your second commit above?In order for page->mapping to get set to NULL, we'd have to be removing the page from the page cache altogether. I'm not seeing where we'd be doing that here. It certainly isn't possible for some third party to do so, since our thread is holding the page lock and I'm not seeing where the call to nfs_write_error() might be doing so. We do call nfs_inode_remove_request(), which removes the struct nfs_page that is tracking the page dirtiness, however it shouldn't ever result in the removal of the pagecache page itself. Am I misreading your email?thanks very much Trond; much more likely I am misreading the code :) My theory was that we have nfs_page_async_flush detecting nfs_error_is_fatal_on_server, so calling nfs_write_error_remove_page (this is an older v4.14.72-ish kernel). That would then generic_error_remove_page -> truncate_inode_page -> truncate_complete_page -> delete_from_page_cache thus, as you say, removing the page from the page cache, with __delete_from_page_cache clearing page->mapping. Without your v5.0 commit, nfs_page_async_flush will then return 0, via nfs_do_writepage, nfs_writepage_locked, nfs_wb_page to nfs_try_to_update_request, which then returns NULL to nfs_setup_write_request. nfs_inode_add_request, which itself then dereferences the mapping: spin_lock(&mapping->private_lock); which is where we crash. Obviously, there are a number of assumptions in the above, so I thought it must just be a possible path the code could take? Does that sound plausible (given that v4.14.72-ish code)? However, I note that in a subsequent v5.2 commit: 22876f540bdf NFS: Don't call generic_error_remove_page() while holding locks you remove the call to generic_error_remove_page from nfs_write_error_remove_page(), and that is itself then renamed nfs_write_error(), as part of a later v5.2 commit: 6fbda89b257f NFS: Replace custom error reporting mechanism with generic one Without those commits, and also without your adjustments to nfs_page_async_flush I mentioned earlier, is it possible that the code path I present above, where the page /is/ removed from the page cache, could result in the crash we saw?OK, yes that is plausible. The call to generic_error_remove_page() would, as you said, remove the page from the page cache, and thus could result in the crash you describe.
that's great, thanks very much indeed for the confirmation, Trond. sorry to waste your time with older code… cheers, calum.
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