Re: [PATCH] bcachefs: Fix deadlocks between fallocate and readahead

[Deepanshu Kartikey]

I found that bchfs_fallocate() only evicts pagecache when FALLOC_FL_ZERO_RANGE is set:

if (mode & FALLOC_FL_ZERO_RANGE) {
    truncate_pagecache_range(&inode->v, offset, end - 1);
}

The hole detection code already skips ZERO_RANGE:

if (!(mode & FALLOC_FL_ZERO_RANGE)) {
    bch2_clamp_data_hole(...);  // Only runs for basic fallocate
}

The syzbot reproducer uses mode=0 (basic fallocate), which doesn't evict pages but does run hole detection and hits the deadlock.

The fix should be to remove pagecache_block_get() from bch2_fallocate_dispatch() and add it only where pages are actually evicted:

In bchfs_fallocate():
if (mode & FALLOC_FL_ZERO_RANGE) {
    bch2_pagecache_block_get(inode);
    truncate_pagecache_range(...);
    // Do allocation work
    bch2_pagecache_block_put(inode);
}

Similarly add it to bchfs_fpunch() and bchfs_fcollapse_finsert() around their page eviction calls.

This way basic fallocate never holds the lock, avoiding the deadlock. Does this approach look right?